Add CI with code coverage using Travis CI and Codecov

[mpeterv]

This adds configuration for Travis CI to run t/test.lua on each commit under Lua 5.1, 5.2, 5.3, and LuaJIT 2.0, 2.1.

It looks like this: https://travis-ci.org/mpeterv/serpent

Additionally, test coverage is collected using Luacov and uploaded to Codecov. Looks like this: https://codecov.io/gh/mpeterv/serpent/branch/ci

Coverage is pretty good, only two if branches are not touched by existing tests.

Also, add nice badges to readme to make it easy to see if the build if failing currently and what is the current coverage percentage.

Before applying this, need to register on travis-ci and add the repo. Also need to join Codecov, but no need to add the repo manually there.

[pkulchenko]

Looks good; thank you for the patch! I'll indeed have to register with travis and codecov as you said; plan on doing this soon.