Added a function to stop the tox_do/toxav_do loop

Also added txdv's contribution to the ToxAv class
Merge branch 'txdv-master'
Add ToxFriend.
Merge branch 'friend' of https://github.com/txdv/SharpTox into new-api
Some minor changes

- Made the constructor in ToxFriend internal
- FriendList is now private
- Other minor stuff
Add initial group chat changes.

Need to take a look on what peers do and how we should create them
Create ToxGroup for event args.
Add ToxGroupPeer and ToxGroupInvite.
Move peerNumberIsOurs.
Add ToxFileSender.
Change signature of FileSendData.

Now it is only SendData in the ToxFileSender object.

Furthermore accepts arbirtrary ranges of arrays.
Add ToxExtensions class.
Add extension for SendData.
Add constructor overload for tox with no parameters.

Uses default parameters.
Change default options for ToxOptions in Tox constructor.
Move IsConnected code to Iterate.

This ensures that the iterate functions works stand alone
just like the the Loop function.
Move GetGroupMemberName to ToxGroup
Add Groups field in Tox instance.
Move GroupPeerPublicKey to ToxGroupPeer
Adjust FileSender event args to hold ToxFileSender.