Improve performance of StringExtensions

StringExtensions.included checks to see if some methods are defined
in a very inefficient manner.

This commit rewrites it to use method_defined?, avoiding tens of
thousands of calls to Symbol#to_s and String#== when requiring
highline (and fixes #91).

Tbh, the code in StringExtensions does seem a bit crufty overall, but I
didn't want to get bogged down.

The test suite still runs fine, and does seem to test this code, but I
must admit to not being very familar with these methods and haven't done
any manual testing of my own. It'd be good if someone could review this
more closely.
Merge pull request #92 from johnl/master

Improve performance of StringExtensions