Lua script binding

This patch provides Lua script bindings and examples.

Documentation at http://librocket.com/wiki/documentation/LuaManual needs to be linked into the website properly in the places where python is mentioned as well.

Most of the work is from @Gambini as originally submitted in PR #49 with minor updates to bring it to match up with the current master tree.

Includes provisions for reusing an existing Lua context allowing the context to be shared with other parts of the applications libRocket is embedded in.