Make methods in SystemExtensions public

This is an attempt to fix https://github.com/JEG2/highline/issues/57

Most of the methods rely on instance variables, so it does not make sense that they should be available as module methods. But when they are included into the HighLine class, they are private, so not useful outside of the class.

Making the methods public doesn't seem to have a negative effect on test results, and it allows the methods to be called on instances of the HighLine class.

This might break some stuff.