feat(ionicContent): use child scope instead of isolate scope

Adds new '$ionicBindParent' service, which takes an object containing
binding definitions (similar to angular directive isolate scope
definition).  Allows binding of any scope and attributes from any parent
scope, letting us do normal parent -> child scope binding without having
to create isolate scopes.

Closes #555.

We could refactor many current isolate scope directives to use this, as well, and fix many inheritance / controller problems.