RDoc documentation

There are 3 commits:

- 8b8816a: Documents files under lib/ and removes docs (with `:nodoc:`) to global methods in extconf.rb.
- baf0c6e: Added a CONTRIBUTING file and tried to clean the README.
- 73c456e: Documents files under ext/ and removes a *lot* of whitespace and comments that separate empty sections.

There are some things I still don't like about it, e.g. RDoc is too verbose and the problem with the `(METHOD)` parsing. Well, I was thinking about [YARD](http://yardoc.org/), but this should be enough for now.

We need to change the template to have a better feel in sciruby.com. And a rake task such that `rake add_docs` would create the correct directories, git clone the NMatrix repo, run the correct RDoc command and make all the necessary commits.

For that matter, the command I'm using at the moment is:

rdoc --main README.rdoc --exclude=./pkg --exclude=./tmp --exclude=./spec --exclude=Rakefile --exclude=Gemfile --exclude=Guardfile --exclude=Manifest --exclude=Makefile --exclude=ext/nmatrix/extconf.rb --exclude=ext/nmatrix/ttable_helper.rb

Also, I rebased it against the current sciruby/master branch, so it should merge without conflicts. (this is my first time using it, so if there's something wrong, please correct me)