Hg storage now based off mercurial module, not 'hg' executible.

Hg storage now based off mercurial module, not 'hg' executible.

This should make repeated calls to Hg storage instances _much_ faster,
since we avoid repeatedly loading and tearing down a python subprocess.
For example, the testsuite runs ~6x faster on my box.

Here's a run with the old Hg implementation:
  $ python test.py libbe.storage.vcs.hg
  ...
  =================================
  ERROR: test_get_previous_children
  ---------------------------------
  Traceback (most recent call last):
    ...
  NotImplementedError

  ---------------------------------
  Ran 49 tests in 133.285s

  FAILED (errors=1)

A run with the new implementation gives the same results, except for:
  Ran 49 tests in 22.328s