add license to gemspec

This way you can get that when using rubygems.org API
Use Erubis for parsing static method bodies if available
Use TinyRuby rather than Eruby for static method parsing.
Make xss mods available standalone

This makes the XssMods module available in a seperate file. This allows
other frameworks that implement the SafeBuffer API to use it without using
ActionView.
Merge pull request #625 from jordimassaguerpla/master

add license information to the gemspec
Merge branch 'master' into 3-2-stable