#6 Allow for different User Agent

[uiltje]

Attached, please find a patch that refactors the implementation of UserAgent such that it can easily be replaced by an alternative user agent implementation.

Basically, this is done by introducing the "UA.java" interface definition.

A completely different/alternative implementation can now be made by simply implementing the UA interface without losing the ability to utilize underlying handlers.

Hope this is useful,
Tom.

[uiltje]