Add `--patch` flag to apply a Ruby patch from stdin

If `-p|--patch` flag was set while invoking `ruby-build` or
`rbenv install`, ruby-build will use `patch -p0 -i -` to apply a patch
from stdin to Ruby, Rubinius, or JRuby source code before running the
rest of `build_package_*` commands.

References #443
Shorten `rbenv install` usage synopsis

It was getting out of hand, so just use the single-character flags which
are documented below anyway.
Clarify which `rbenv install` flags get forwarded to `ruby-build`
Add ruby-2.1.0-rc1.
Merge pull request #474 from havenwood/2.1.0-rc1

Add ruby-2.1.0-rc1.
Add examples how to use the `--patch` option to the Readme

[ci skip]
Merge branch 'apply-patch'

Closes #469
Transform JRuby shebangs to absolute paths, fixing recursion within rbenv

The default shebang for binstubs in a JRuby installation is
`#!/usr/bin/env jruby`. On a rbenv-managed JRuby, this will execute the
`jruby` rbenv shim. In a situation where a JRuby tool like `gem` is
executed from within a running rbenv by a rbenv plugin, (e.g.
rbenv-gemset) this can trigger another rbenv run to resolve the shim.
This can lead to endless recursion of rbenv calls.

Fixes #471, closes #473
References jf/rbenv-gemset#56
ruby-build 20131220
Fix JRuby install on systems with non-BSD `sed`

There doesn't seem to be a way to construct a sed invocation for
in-place editing that is compatible with both BSD and GNU sed.

http://stackoverflow.com/q/5694228/11687

Fixes #475
Thorougly test JRuby install process
Merge branch 'jruby-sed'

Closes #476
ruby-build 20131220.1
preparation of Ruby 2.1
use official ruby repository
Merge pull request #477 from hsbt/heroku-support

Added definitions of Heroku's backports.
added md5
Merge pull request #479 from hsbt/release-2-1-0

Definitions of Ruby 2.1
Unify tense in CHANGELOG
Rename 2.1.0 -> 2.1.0-p0