Include the parsed XML tuple in the pjsip_pres_status, and include it in the pjsua_buddy_info in PJSUA-LIB, in case the PIDF document contains other info that is needed by application (thanks Johan Lantz for the suggestion)

[bennylp]

Several enhancements on the presence body handling:

• include the full parsed XML tuple in pjsip_pres_status structure, as it may contain additional information for the application
• include pjsua_pres_status in pjsua_buddy_info so that this information is accessible by PJSUA-LIB application.
• add timestamp info in the outgoing PIDF. There may be case when subscriber receives PIDF with multiple tuples, and in this case it can use the timestamp to decide which tuple to use.
• also when we receive NOTIFY with empty body, notify the application about the event, keep the presentity status unchanged, but set the tuple pointer in pjsip_pres_status structure to NULL.

[bennylp]

Done in r2762:

• implements all that are specified in the ticket description above
• reduced memory usage in the session. Previously, memory will grow everytime presence subscription (server and client) is updated, and this memory will only be freed when the subscription is terminated

[nanang]

In r2764:

• Minor update: cleaned up compile warnings of undefined function.