Build OpenSSL so Ruby 2.0 installs Just Work on Mountain Lion

Ruby 2.0 won't compile the OpenSSL extension on Mountain Lion because Apple shipped a crippled OpenSSL distribution. The fix is to build your own OpenSSL and link to that instead:

```
  brew install openssl
  CONFIGURE_OPTS="--with-openssl-dir=`brew --prefix openssl`" \
    rbenv install 2.0.0-rc1
```

That's a lot of hassle for what will soon be a very common case. We can fix it by bundling a working OpenSSL.

TODO:
- [x] The OpenSSL build needs to be limited to Mountain Lions using the crippled distribution.
- [x] CONFIGURE_ARGS is hacked.
- [x] And the OpenSSL configuration options need review.
- [x] set `--openssldir` to `$PREFIX/openssl/ssl`
- [x] use a subpath for openssl install so we don't get an openssl shim for its bin/openssl
- [x] provide root certs: http get http://curl.haxx.se/ca/cacert.pem > OPENSSLDIR/certs/cacert.pem
- [x] `c_rehash` certs so openssl sees the new cacert.pem
- [x] do a post-install check that the ruby openssl extension installed, since it will silently continue otherwise
- [x] extract openssl bundling and verification to the main executable so other definitions can reuse them
- [x] update 2.0.0-dev and -preview definitions to bundle openssl
- [x] dump certs from system keychain rather than download from curl's site
- [x] use homebrew openssl if available. quicker than rebuilding for every ruby install.
- [x] deal with embedded quoting in --configure args; need to switch to an array