generateOffer and CreateDataChannel APIs

[mo3rfan]

In order to facilitate a connection between two Cpp clients instead of the browser, we've added PeerConnection::GenerateOffer() and PeerConnection::CreateDataChannel(std::string label, std::string protocol)

To see it in action, run the websockets server, followed by testclientanswer in one terminal and testclientoffer in another, in that order.

We've also included some general fixes that we've come across:

• Fix string comparison in ParseOffer (https://github.com/hamon-in/librtcdcpp/commit/0ae722c4ab7de706fd3609ea9ca6ad4fa0dcb7ce)
• DCEP structure alignment fix.

[chadnickbok]

Howdy; did you want me to take a look at this, or are you closing while some more changes are made?

[mo3rfan]

I closed this to make some more changes. Will reopen once it's ready :)

[mo3rfan]

~~I think this should be all for this PR to be feature complete and functional.~~

EDIT: Totally forgot that I hardcoded a few stuff and probably need to add a few things for the SCTP layer on handling different channel types.

[chadnickbok]

Awesome, thanks for sharing all this hard work!

I'll try to take a look at this tonight and get back to you asap.