#3 Move UserAgent to core package

[Brent]

UserAgent is commonly used and a very useful part of jjsip. It should probably be in the main package, rather than the "reference implementation" package.

[Brent]

Along with this, the audio stuff in UserAgent should probably be moved to a new sub-class "GraphicalUserAgent" that extends UserAgent. UserAgent should be generalized to provide the common functionality for both graphical and command-line (text) UAs.

[Brent]

local.ua.UserAgent will need to be modified to extend the new UserAgent class, adding the media stuff.