Update NMatrix documentation

I updated the documentation from various parts of the library and also:
  - Changed inclusion/exclusion config of rdoc task.
  - Added `:nodoc:` in internal things (e.g. GC-related code, monkey
    patching RSpec).
  - I also removed autoloads and converted them into `require`s, which
    are all placed in lib/nmatrix.rb (where they should be!).

Hide constants with :nodoc:

`NMGCHolder` class is an important part of NMatrix's internals, but
it should definitely be hidden in the generated documentation.

The RSpec extension (to `BeWithin`) is only used during testing and
shouldn't be shown on the documentation. I selectively excluded its file
in the Rakefile's RDoc task.

Lots of other classes and modules were also :nodoc:'d.

Documentation for YALE_GROWTH_CONSTANT

Using C-style comments make for better RDoc style. And not including the
constant's name in the `Document-const` directive turns out to generate
a cleaner documentation.

More documentation for NMatrix::[] / N::[]

I just added one more example and made the call-seq: instruction a bit
more informative.

I also removed :nodoc: directives from `ttable_helper.rb` and removed
the file from RDoc generation. It seems cleaner that way.

Add documentation to stype shortcuts

Add documentation for specialized concat methods

Remove unnecessary :nodoc:s

It's much easier to remove files from RDoc generation instead of adding
`:nodoc:` to every method, class and module in it.

Add some documentation to the NMatrix class

Documenting NMatrix::IO modules and classes

The most important parts for the public are the modules for loading
matrices from specific formats -- MatrixMarket, PCD and Matlab. It isn't
necessary to show their internals, which just pollute the API
documentation.

I also removed autoloads for NMatrix::IO classes and put `require`s in
`lib/nmatrix.rb`. This is easier and the speed difference isn't really
relevant for us -- those extra modules aren't going to significantly
slow down a program.

Fix description of NMatrix class

Re-insert comments

I don't think the comment showing where the class "ends" is useful, but
no problem. The other two were mistakes: we should keep internal docs
too! :P

Btw, I moved the autoloads to lib/nmatrix.rb so we have only `require`s
there.

Another comment reinserted

Fix docs in nmatrix/shortcuts.rb

Allow docs in NMatrix::Matlab

Reinsert :nodoc:'s