mongo rewrite

[buhman]

Fixes #99.

FIXMEs related to:

• cache invalidation (no location header in put/delete--this has actually always been broken in various ways)
• broken tests that need to be rewritten

All other functionality ShouldWork™.

I don't consider this to be merge/deployment-blocking, but they do need new issues opened afterwards.

[buhman]

Test coverage before rewrite:

Name                                                         Stmts   Miss  Cover
--------------------------------------------------------------------------------
.tox/py34/lib/python3.4/site-packages/pb/__init__                0      0   100%
.tox/py34/lib/python3.4/site-packages/pb/cache                  47      1    98%
.tox/py34/lib/python3.4/site-packages/pb/db                     20      0   100%
.tox/py34/lib/python3.4/site-packages/pb/paste/__init__          0      0   100%
.tox/py34/lib/python3.4/site-packages/pb/paste/handler          12      0   100%
.tox/py34/lib/python3.4/site-packages/pb/paste/model            52      2    96%
.tox/py34/lib/python3.4/site-packages/pb/paste/views           103      0   100%
.tox/py34/lib/python3.4/site-packages/pb/pb                     73      7    90%
.tox/py34/lib/python3.4/site-packages/pb/url/__init__            0      0   100%
.tox/py34/lib/python3.4/site-packages/pb/url/model              15      0   100%
.tox/py34/lib/python3.4/site-packages/pb/url/views              25      0   100%
.tox/py34/lib/python3.4/site-packages/pb/util                   59      0   100%
--------------------------------------------------------------------------------
TOTAL                                                          406     10    98%

1 failed, 16 passed in 7.98 seconds

Test coverage after rewrite:

Name                                                        Stmts   Miss  Cover
-------------------------------------------------------------------------------
.tox/py34/lib/python3.4/site-packages/pb/__init__               0      0   100%
.tox/py34/lib/python3.4/site-packages/pb/cache                 47      1    98%
.tox/py34/lib/python3.4/site-packages/pb/db                    14      0   100%
.tox/py34/lib/python3.4/site-packages/pb/paste/__init__         0      0   100%
.tox/py34/lib/python3.4/site-packages/pb/paste/handler         12      6    50%
.tox/py34/lib/python3.4/site-packages/pb/paste/model           20      0   100%
.tox/py34/lib/python3.4/site-packages/pb/paste/views           97      7    93%
.tox/py34/lib/python3.4/site-packages/pb/pb                    78     14    82%
.tox/py34/lib/python3.4/site-packages/pb/url/__init__           0      0   100%
.tox/py34/lib/python3.4/site-packages/pb/url/model             14      0   100%
.tox/py34/lib/python3.4/site-packages/pb/url/views             23      1    96%
.tox/py34/lib/python3.4/site-packages/pb/util                  50     13    74%
-------------------------------------------------------------------------------
TOTAL                                                         355     42    88%

2 failed, 15 passed in 0.74 seconds

[buhman]

deployed