Removed extending call handler
Moved the call hanlder interface to its own section (closes #38)
Added TelephonyCall implements CallHandler, fixed headings
removed redundant text about CallHandler objects
Linked to implement definition in WebIDL
Fixed headings, moved implements IDL statements
removed mention of CallHandler
fixed merge conflict
Merge pull request #41 from marcoscaceres/implement_callhandler

Implement callhandler (closes #23, #27, #30)