Code editor using QScintilla

This pull request adds a new qgscodeeditor object which can be used as a base for all text editors that need good editor support and syntax highlighting, project macros, expression widget, html editors, etc

Original Code by Salvatore Larosa

Tested on Windows.  Needs testing on other platforms.

![image](https://cloud.githubusercontent.com/assets/381660/3594260/21d26986-0c98-11e4-9d6c-491af2b68c1f.png)