Added timer support to PJSIPUA

Ignore-this: 807fb506c5ac20e2c5db69b3964b75a7

darcs-hash:20091005193548-1e3d8-81178fd67ea9f6c4c52c7e2135da6eeb672ad3f3
Fixed session to update streams according to SDP answer in reINVITE answer

Ignore-this: bf9aba5367ed3b1c6bd58145465693bd

darcs-hash:20091007141917-1e3d8-521529fe3c943b4227879271da7c095420e7e52a