Add Travis continuous integration support.

Build logs are available here (authenticates with your GitHub account):
https://travis-ci.org/laas/metapod

Current configuration is building master branch *only*
with both gcc and clang while sending notifications to hpp-source@laas.fr
if necessary.

README.md is modified to display the current build status (success or
failing) of the last commit.

See Travis documentation for more information:
http://about.travis-ci.org/docs/