Added a sanity test to Travis-CI build process

Added a simple sanity test that runs after Travis-CI completes a successful compile.

The Python [pexpect](http://pexpect.readthedocs.org/en/latest/) module is used to interact programmatically with Unix-CB.

The test ensures that:
- `cb/cbinit` runs successfully
- `cb/cb` prints out welcome banner
- `cb/cb` successfully asks for user's handle and logs user in
- `cb/cb` echoes back a message that the user sends