add .travis.yml for continuous integration
add .travis.yml for continuous integration

This configuration verifies:

• that code is syntactically correct (most interesting for pull
  requests)
• that code was gofmt'ed
• that code compiles with go 1.4, 1.5, 1.6 and 1.7
• that tests pass with go 1.7

Debian/debiman,stapelberg/debiman,stapelberg/debiman