Added Cocoapods podspec 

Added from Specs-master/Specs/MMMarkdown/0.3/MMMarkdown.podspec.json.

This allows those of us using cocoapods to manage dependencies to pull in the latest master commit of MMMarkdown.  In order to to do that, the pod spec needs to be in the root of the repo as per http://guides.cocoapods.org/using/the-podfile.html#from-a-podspec-in-the-root-of-a-library-repo

All I did was copy the existing pod spec from the Cocoapods master spec repo and put it in the root of the MMMarkdown repo.  This allows me to use MMMarkdown in my project in one of two ways:

````
# Use the latest GM release from cocoapods (currently version 0.3)
pod 'MMMarkdown'

# or use the latest commit to master
pod 'MMMarkdown', :git => 'https://github.com/mdiep/MMMarkdown.git'
````

I have tested both after the change and they both work so this is a low-risk merge.

Thanks for MMMarkdown - I'm really enjoying using it!  :-)