Updated .rvm to ruby-2.0.0 and added yard gem as dev depenency

I was using ruby-1.9.3 as the previous .rvmrc file recommended, but
then I tried to install yard to generate local docs, and it failed
with a "Missing 'redcarpet' gem" error, that does not happen using
ruby-2.0.0, so I assume the yard gem got updated or improved for
ruby-2.0.0 and so it seemed a good hint to update .rvmrc to use
ruby-2.0.0, as probably ruby-1.9.3 will get more and more outdated
with less support in updated gems, or so I thought :)
Merge branch 'rvm_use_ruby2.0.0'