Support animating UIEdgeInsets on iOS

Adds basic support for UIEdgeInsets. Currently only supporting iOS because edge
insets aren't as common to work with on OS X (e.g. NSScrollView doesn't have
a contentInset property).

Added tests and verified they all pass on the 32- and 64-bit iOS simulators and
under the pop-osx project as well.
Merge pull request #128 from ide/insets

Support animating UIEdgeInsets on iOS