Fix RVM recipe

**Note:** This probably depends on #131, so **merge that first**

 - Fix multiple issues:
   - Fixes #124
   - Fixes #5
   - Workaround
     [CHEF-3940](https://tickets.opscode.com/browse/CHEF-3940) for
     chef-solo
 - Set `rvm_make_flags` based on number of CPUs
 - Added attribute `node["rvm"]["multithread_build"] = true`
 - Create default .rvmrc for default settings
 - Now uses official installation method
 - Version string must be specified as one of: `latest|latest-minor|latest-x|latest-x.y|x.y.z`



Recipe Documentation:
----------------------------------

To turn off multithreaded build:

```yaml
recipes:
    - pivotal_workstation::rvm
    node_attributes:
      rvm:
        multithread_build: false
```

To set custom compile options & install other ruby versions:

```yaml
recipes:
    - pivotal_workstation::rvm
    node_attributes:
      rvm:
        default_ruby: ruby-1.9.3-p392
        rubies:
          ruby-1.9.3-p392:
            :command_line_options: --verify-downloads 1  --enable-shared  --with-openssl-dir=$rvm_path/usr --with-readline-dir=$rvm_path/usr
          ruby-1.9.3-p448:
            :command_line_options: --verify-downloads 1  --enable-shared  --with-openssl-dir=$rvm_path/usr --with-readline-dir=$rvm_path/usr
```

To install another version of RVM, specify using one of `latest|latest-minor|latest-x|latest-x.y|x.y.z`:

```yaml
recipes:
    - pivotal_workstation::rvm
    node_attributes:
      versions:
        rvm: 1.21.20
```