TwDocument, TwElement fixes and Unit Tests

* Added TwDocument Collection tests
* Added TwDocument tests
* Added TwElement tests
* Improved tests in ObjectModel
* TwElement contents is calculated from GXml.Text nodes, setting
  its contents, removes all of them and adds just one with new
  content
* Fixed TwDocument's TwElement's contents writing
* TwDocument's node names defaults to #document
* Improve configure resume
* Disabled performance tests by default --enable-performance-tests
  to enable them