Parse Organization

I've made an attempt (maybe naive?) to organize the Parse class a little better. I've sorted and reorganized the combinators across a few new files, and alphabetized them in their files. I put the parser helpers into sub-classes Numerics, Characters, and Parsers, and I've put everything in the Parse class into a single folder.

Some other changes I've made:
-I added a version of the parser for an identifier to Parse.Parsers.
-Added a couple convenience parsers: a version of Contained to accept one parser as the open and close, and a version of Then to accept just a parser when the Func is: a => second.Return(a).
-Updated code around the project to accept changes to Parse class.

I also wrote versions of the Parse and TryParse extensions to accept a TextReader in lieu of a string, but they just read the TextReader to the end and pass that string along as input, which isn't really what you're going for in your earlier discussions, but it was all I needed when I put them in - possibly not keep them?

Obviously I'm a bit of a newbie to GitHub, but I'd love any feedback on what I've got here, and I'd be excited if I could help this project!