Use official mozinstall package to get applications installed/uninstalled  (#127)

This pull request attempts to fix issue #127.
This line fails for me on Linux when I try to run the testrun script against an already installed version of Firefox like `testrun_functional /mozilla/bin/nightly/firefox`:

Traceback (most recent call last):
  File "/home/henrik/.virtualenvs/mad/bin/testrun_functional", line 9, in <module>
    load_entry_point('mozmill-automation==2.1-dev', 'console_scripts', 'testrun_functional')()
  File "/mozilla/code/mozmill-automation/mozmill_automation/testrun.py", line 800, in functional_cli
    exec_testrun(FunctionalTestRun)
  File "/mozilla/code/mozmill-automation/mozmill_automation/testrun.py", line 783, in exec_testrun
    cls().run()
  File "/mozilla/code/mozmill-automation/mozmill_automation/testrun.py", line 359, in run
    if mozinstall.is_installer(self.binary):
  File "/home/henrik/.virtualenvs/mad/local/lib/python2.7/site-packages/mozInstall-1.10-py2.7.egg/mozinstall/mozinstall.py", line 151, in is_installer
    src = os.path.realpath(src)
  File "/home/henrik/.virtualenvs/mad/lib/python2.7/posixpath.py", line 382, in realpath
    path, ok = _joinrealpath('', filename, {})
  File "/home/henrik/.virtualenvs/mad/lib/python2.7/posixpath.py", line 388, in _joinrealpath
    if isabs(rest):
  File "/home/henrik/.virtualenvs/mad/lib/python2.7/posixpath.py", line 61, in isabs
    return s.startswith('/')
AttributeError: 'NoneType' object has no attribute 'startswith'