Rename Apptrace to Appdash.

[slimsag]

Rename the Apptrace project to Appdash.

Notes:

• All tests pass, and cmd/appdash runs OK.
• Travis will fail until sourcegraph.com/sourcegraph/apptrace's go-import meta-tag and this repo are renamed.
• httptrace sqltrace and traceapp packages keep their names.

Breaking changes (important!):

• Import path sourcegraph.com/sourcegraph/apptrace renamed to sourcegraph.com/sourcegraph/appdash.
• Tool cmd/apptrace renamed to cmd/appdash.
• /tmp/apptrace.gob renamed to /tmp/appdash.gob.
• The WWW-Authenticate header is now set to Basic realm="appdash" instead of Basic realm="apptrace".

@sqs Please take a look -- what are your thoughts?

[sqs]

Merged, thanks!

Travis CI has cached some old clone, so the merge commit is failing (despite multiple retries). But I pushed a new empty commit to a new branch and it worked. So future commits or PRs should pass Travis.