Prefix selector caches with a dollar symbol

to help identify them as such later on in the code.

This is a fairly common jQuery pattern:

http://stackoverflow.com/questions/6209462/when-why-to-prefix-variables-with-when-using-jquery
http://encosia.com/5-steps-toward-jquery-mastery/
Merge pull request #74 from Sid3Show/master

Prefix selector caches with a dollar symbol