Added tasks to allow rbenv setup, update and rubies installation.

Setting up rbenv is a pretty trivial task so I added a couple of tasks to help manage this.

This might not be the case for everyone, but I thought I should propose first before putting it in a separate gem.

There are 3 new tasks and 3 new config directives:
* `rbenv:setup` - installs rbenv remotely
* `rbenv:update` - updates the rbenv installation
* `rbenv:install[RUBY_VERSION]` - installs a rubie, in case you want to upgrade the ruby version

* `ruby_build_path` - this will use ruby build to manage rubies installation, this option is where it should be installed, defaults to `$RBENV_PATH/plugins/ruby-build`
* `rbenv_git_url` - the git url to use for setup, defaults to @sstephenson/rbenv
* `ruby_build_git_url` - the git url to use for installing ruby-build, defaults to @sstephenson/ruby-build

Thanks.