Added custom variable class
Added functions for getting and adding custom variables to Tracker
Added test case for custom variables
Updated custome variables to view helper output.
Added test for custom variables.
Set _setCustomVar before _trackPageview
Changed validation of scope variable to use scope constants
Fixed bug with adding custom variables. Added unit test for exception when adding variables with same index.
Remove commented line about deprecated clearing
Merge branch 'feature/custom-variables'