Fixes here and there

Core:

Fixed problem reading identity which caused reg error
Keep connection alive through perioding pinging, thanks to @galmeida
Fixed compatibility with the whole python 2.6-3.4 spectrum
Added StackBuilder to make stack construction easier

Protocol:

Distinguish received delivered and read receipt, thanks to @NickCis
Fixed leave groups, thanks to @menachem.shapira@tandemg.com
Added Profile Layer
Add participants to a group, thanks to @emamirazavi
Fixes to registration, thanks to @aesedepece
Updated S40 registration token
Save next challenge and use in next auth, thanks to @samiexa
Get contact profile picture, thanks to Jamie Gloudon

yowsup-cli and demos:

Fixed groups jid handling in sendclient, thanks to @fire855
Fixed readline redundant dependency in linux, thanks to @NickCis
Added group_invite in cli demo to add participants to a group, thanks to @emamirazavi
Added leave_group to cli demo