[ANN] Travis CI continuous integration

[pfalcon]

Automated build checks.

This is long overdue, but better late than never. Original work on Travis CI build and config is due to https://github.com/SuperHouse/esp-open-rtos

As the build takes quite a lot of time, and Travis offers their services for free, I'm still too shy to do that in master for each commit, so for now that lives in the "travis" branch (i.e. needs manual steps to trigger): https://github.com/pfalcon/esp-open-sdk/commits/travis