feat(filters): implement isState and includedByState filters

Some global filters to help mitigate the writing of really bad code
for ng-class expressions, among other things, in templates.

Usage:

"stateName" | includedByState --- Translates to $state.includes("stateName")
"stateName" | isState --- Translates to $state.is("stateName")

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Such filters are trivial for an application to implement themselves, however
people seem to not do this and prefer to write horrible verbose expressions
instead --- Lets encourage the use of a powerful AngularJS feature instead.
Merge pull request #627 from caitp/issue-627

Global filters for $state.is() / $state.includes()