Documentation on ruby land.

Made extensive changes to how documentation is organized. Mainly changed comments, but some method's APIs were altered too. Added :nodoc:'s in lots of files, that's why there are changes in spec/ and ext/.

I've changed the NMatrix::VERSION to a module, following Rails convention.

Finally, almost everything has a comment in it, but there are lots of writing to be made - blas.rb and lapack.rb aren't finished yet and need a better explanation of the arguments for each method. Introduction of hashed parameters in them would be great.

NMatrix/NVector/shortcuts coverage is pretty good.

Fixed a bug in slice_spec
Changes in README, addition of CONTRIBUTING file.

README was reorganized to present usage information and examples first. The CONTRIBUTING.md file is a guide on how to contribute to SciRuby and, specifically, to NMAtrix.

The only changes in LICENSE.txt and History.txt are dates. We're in 2013, remember. :)
Added docs; cleaned other comments.

Added documentation for rb_define_methods in nmatrix.cpp and storage/yale.cpp. I'm using the #ifdef 0 ... #endif trick, but I'll patch RDoc (or use YARD) to avoid this in the future.

Also cleaned the comments a lot -- mainly removed "section" separators for empty sections. Removed some extra newlines, too.
Fixed some mistakes in README