Add Travis.
Add Travis build status to README.md.
Check style before running test on travis.

Currently, travis just runs tests.

This patch makes travis check style first, and run test only when it
passes style check.
Change check level from warning to error.

To check whether style is suitable with a command result, all warnings
have to be changed to erros in scalastyle. Because scalastyle returns
zero even though there are warnings.