[FEATURE] Add comment functionality to QgsExpression
Add block comment tests
Preserve expression formatting
QgsExpression: some const fixes
Add line comments to QgsExpression

Delimited by double-minus and EOL/EOF

Example:
  $id -- This is the feature id
Change unary minus test --1 to -(-1)

See http://lists.osgeo.org/pipermail/qgis-developer/2014-November/035835.html