Twitter 1.1 API (now required), %-encoding bugfix

Changes for the Twitter 1.1 API (1.0 is disabled by Twitter!)
- remove (some) XML references. 1.1 does not support XML anymore!
- update twitter domain to 1.1
- removed LibXML files
- moderate documentation updates, fix compiler warning in MGTwitterXMLParser.h

Note: not all REST commands have been tested for 1.1 compliance.