Add support for travis-ci service

This allows running tests on shared continuous integration testing
service travis-ci.org. Currently it does:
1. compile
2. run tests (incl. ones that require protobuffers)
3. run erlang tests

Tests are executed on Erlang platforms R14B02-R16B.

Here, it works: https://travis-ci.org/Motiejus/piqi

Steps which should be done before merging this pull request:
1. log in to travis-ci.org with your github account
2. find "piqi" in "my repositories"
3. click "on"

Then merge. Once the pull request is merged, github bot will notify travis-ci that something has been updated, and piqi will be tested.

When piqi-erlang and piqi-rpc are completely decoupled from this repository, this will obviously change.