Use OS-supplied `stat` on OSX.

Re: postmodern/ruby-install#63
Merge pull request #64 from paul/master

Use OS-supplied `stat` on OSX.
Do not silence stderr.
No need to wrap command -v in [[ ]].
Prefix all functions with function.
Use "$( )" over ` `.
Use ${var%/*} instead of dirname and ${var##*/} instead of basename.
Use (( )) for arithmetic expressions.
Add failing spec for CONFIGURE_OPTS word-splitting
Fix mis-quoting of CONFIGURE_OPTS

This fixes cases like:

    ruby-install ruby 2.0.0 -- CFLAGS="-march=x86-64 -O2"

Which were getting incorrectly word-split into two arguments.
Use * not @ when passing options to ./configure
Do not assume homebrew is installed at /usr/local/bin/brew. (closes #66)
Merge pull request #65 from pbrisbin/master

Fix mis-quoting of CONFIGURE_OPTS
Fix additional option parsing bug
Merge pull request #67 from pbrisbin/master

Fix additional option parsing bug
use CDN hosts in default setting
Merge pull request #69 from hsbt/use-cdn

use CDN hosts in default setting
Upgrade to rubygems-2.1.0.