New 'inline' callback functions

In many situations, the called callback function does not need to access or modify SipApp's state. In this case, the callback function could be called in-process within the call process. This "inline" functions have also the ability to reply its return value inmediatly, synchronously with the call (but blocking the call process). These two characteristics allow for a much quicker call processing if inline functions are used.

Now, most callback functions allow a version without the SipApp's state argument. See inline_test.erl, sipapp_inline_endpoint.erl and sip app_inline_server.erl for an example.

A full rewrite of the callback function process (inline or not) has also been done. Hopefully the code is much simpler now.

Callback functions which must return synchronously have no longer a "From" argument (currently, get_user_pass/2,3).