Allow plugins to run code before and after the installation process
`rbenv install` with no arguments defaults to `rbenv local`
Confirm reinstalling already-installed versions unless invoked with --force
Move MAKE default near the others
Ensure $MAKE is quoted
Document $MAKE
ruby-build 20130129
Add support for `RUBY_CONFIGURE_OPTS` and `RUBY_MAKE_OPTS`

These variables let you pass Ruby-specific configure and make options
that will not be passed to dependent packages like libyaml.
Add 1.9.3-p385
Merge pull request #277 from sorah/193p385

Add 1.9.3-p385
Update examples to use latest MRI
ruby-build 20130206