Add support for AWS shared credentials files.

[Julian]

Matches the order boto uses:

http://boto3.readthedocs.io/en/latest/guide/configuration.html

[Julian]

This doesn't add all of those, mostly because I am lazy, and all I need is one way to load off the FS.

[codecov[bot]]

Codecov Report

Merging #52 into master will decrease coverage by 0.42%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   96.27%   95.85%   -0.43%     
==========================================
  Files          74       74              
  Lines        8701     8726      +25     
  Branches      609      610       +1     
==========================================
- Hits         8377     8364      -13     
- Misses        202      231      +29     
- Partials      122      131       +9

Impacted Files	Coverage Δ
txaws/testing/base.py	94.73% <ø> (ø)	:arrow_up:
txaws/tests/test_credentials.py	100% <100%> (ø)	:arrow_up:
txaws/credentials.py	93.75% <100%> (+2.08%)	:arrow_up:
txaws/route53/client.py	87.5% <0%> (-12.5%)	:arrow_down:
txaws/testing/integration.py	78.57% <0%> (-7.15%)	:arrow_down:
txaws/service.py	87.61% <0%> (-6.67%)	:arrow_down:
txaws/client/ssl.py	71.25% <0%> (-3.75%)	:arrow_down:
txaws/client/base.py	88.17% <0%> (-2.03%)	:arrow_down:
txaws/s3/client.py	94.08% <0%> (-0.81%)	:arrow_down:
txaws/testing/route53_tests.py	99.34% <0%> (-0.66%)	:arrow_down:

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[Julian]

OK, added a fragment I think.