- Made "WEBROOT" more flexible; it's now able to be set through a setDocroot method.
- Removed $_GET dependency and unpredictable alteration of $_GET variables.
- Added setFiles method to set file path(s) without needing $_GET['files'].
- HTTP headers are no longer set directly through PHP's header() function, but instead go through a (configurable) HeaderSetter class or subclass.
- Due to the addition of 'docroot', the CSS _fixRelativeImagePaths had to be changed to prevent bugs when LESS/other files were outside of the web root - though I don't see why this method is needed at all.
- Updated unit tests as applicable to the above changes.

Signed-off-by: tcopestake <terence.copestake@gmail.com>