Move extension source code to ext/ subdirectory

This avoids a potential installation problem where the "install" program
sometimes refuses to install a file (pgplot.so in this case) on top of
itself.  This can happen when using rubygems to install.
Clean up README and FuncUsage for prettier output

Cleans up the RDoc formatting of README and FuncUsage so that the
output generated by RDoc (e.g. when installing via rubygems) looks
nicer.
Mention Ruby 2.0.x support in README