Code coverage support

Support code coverage reports generation via manual and CI builds, publish stats to coveralls.

TODO:
- [x] Simple covegage generation
- [x] Upload coverage reportes to coveralls.io
- [x] Simplify and rewrite gcov.m4 into my_coverage.m4
- [x] ~~Support for OS X~~ will be done after travis-ci/travis-ci#2320

NB: https://vilimpoc.org/blog/2013/07/19/libevent-gcov-lcov-and-os-x/