moved iOS/EmailComposer and iOS/EmailComposerWithAttachments plugins to outside repo

These two iOS plugins have been merged into a single repo @ https://github.com/steve-jansen/cordova-ios-emailcomposer

- new features / changes:
  * plugman/cordova-cli support
  * cordova plugin spec 2.x+ implement
  * ARC support
  * improved JS API
  * unit tests
  * and more, see see https://github.com/steve-jansen/cordova-ios-emailcomposer/README.md