Refactoring gestures.PhoneGesture

Makes use of "extensible enum pattern" so that third party
clients can easily create new gesture types without having
to modify the DroidAR library.

See also: [blogs.oracle.comdarcy/entry/enums_and_mixins](https://blogs.oracle.com/darcy/entry/enums_and_mixins)

Note: this is a breaking change. Since `PhoneGesture`s
are a very new part of the library and extensibility was not
possible before this commit, we suspect that not many
clients have adopted this sensor yet, though.

Signed-off-by: Martin Matysiak <kaktus621@gmail.com>