Allow users to easily add their own app classes

Hi,

this adds the "--imports" command line option. It takes a colon-separated list of Python module names. These will get imported in services/leap.py so that every class derived from LEAPfactory defined in these files gets added as an app. E.g. if you have a file "foo.py" in your $PYTHONPATH which defines "class Foo(LEAPfactory)" and run "leapcast --imports foo" you'll get the Foo app added automaticlly.

This allows developers to easily test their ChromeCast apps without having to modify the Leapcast source code directly.

Would be nice if this could be added.

Bye (and happy new year :-),
Sven