use correct constant instead of variable

http://www.concrete5.org/developers/bugs/5-6-1-2/gravatar-size-in-concreteavatarhelperoutputuseravatar/
Merge pull request #1175 from Remo/patch-49

use correct constant instead of variable
added a bunch of comments, fixed some formatting, improved validation of request parameters
Merge branch 'master' of github.com:concrete5/concrete5