Added 'motion gem' command

Hello,

I've spent my Sunday on this, when I should've been hanging with my family, so please be kind when commenting/reviewing :smile:

The idea is to introduce `motion gem gem-name` command. With the optional switch of `--os=[ios|osx]`. 


I'm doing that by relying on the already existing command provided by Bundler: `bundle gem gem-name`, as it makes sense to use the 'default' way of creating gems. My PR only adds very few RubyMotion specific things/changes to the flow. 

I'm aware that `motion create gem-name --template=gem` exists, but if you look at the generated output, it only caters for **iOS**. Also, the gem template keeps an existing set of files that Bundler would create anyways. So why have that duplication? Rather use Bundler's output and enhance it a bit.

This whole thing started off when I wanted to create a gem for RubyMotion and figured that using:

`motion gem gem-name`

instead of 

`bundler gem gem-name` and look around for what needs to be changed/added for RubyMotion. Admittedly not much, so why not have a command like this that sets it all up for you from the get go?

I did mention that this command targets both iOS & OS X, but I didn't add the OS X relevant files just yet. This is mostly because I'd like to hear what you think, before proceeding further. Also, I'm hungry and need to get some food in me.

Thanks for your time in advance :hamster: 
 
P.S. What could be better ?
When invoking `bundle gem gem-name` use **Open3** to capture stdout, stderr... and then omit the very last line from Bundler: **Initializing git repo in ...** and only have that output at the end. But that is cosmetic.