Added tests for bazaar high-level functions.
Print test name when starting.
Makes it easier to tell when something fails.
Improved tests.

- added two more tests for cases where target exists but is a plain directory
  (not a branch/tree)
- colorize message indicating start of test case, to make it more visible
- fixed some missing imports
- fixed tests for update when target exists, they should work against existing
  tree, not a plain directory
Implemented support for Bazaar VCS.
Both low level functions and the high-level require working_copy interface.
Only run local test if explicitly enabled.
This currently requires inputting the password interactively (there's no way
to tell bzr to use vagrant's key at the moment), so will not work in automated
tests.