Edit pages through the interface.

This PR adds the ability to edit files through the interface. It's an initial implementation, obviously limited and possibly with some rough edges to trim, but mostly solid for what it does.

### Changes:

- Adds `Wiki::editAction`
- Adds `Wiki::_pathIsSafe( string $path )` to verify if a file path is safe to work with (is it within the library? Is it readable? etc)
- Adds `ENABLE_EDITING` configuration constant to toggle the feature on and off ( **off** by default )
- Adds editing interface built-in to the `Toggle source` view

### Preview:

![Selection_001](https://f.cloud.github.com/assets/382538/243664/6b20d11c-8a4f-11e2-8800-7cb489642527.png)

### Side-effects:

- I'm calling this a side-effect for now, instead of bug, depending on what you believe to be the best approach: currently viewing a code snippet will already display the "Save changes" button by default.