Add codecov support

[rpless]

Add code coverage reporting (via Codecov.io)
Notes:

• If you choose to accept this PR then you will need to log into codecov before accepting so that it can set itself up to do reporting
• This uses a slightly out of date version of scoverage because of Support cross-build for Scala 2.10 #18. the latests scoverage does not seem to work with 2.10. if you decide to drop 2.10 support then we can update to 1.3.5

related: #17

see codecov report for my branch: https://codecov.io/gh/rpless/featherbed/branch/feature%2Frpless%2Fcodecov

[codecov-io]

Current coverage is 62.45% (diff: 100%)

No coverage report found for master at 2023cfc.

Powered by Codecov. Last update 2023cfc...7da5803

[rpless]

Or I guess my first bullet is incorrect and codecov will accept anything from a public repo.

[jeremyrsmith]

Thanks for doing this @rpless!