Feature/cocoa pods support

[krodak]

I prepared full support of CocoaPods - the support is added on 2 levels:

• CocoaMarkdown itself uses CocoaPods to maintain its dependencies instead of using git submodules or adding subprojects. This is most common way of adding 3rd party libs to Cocoa projects now.
  I chose to add Pods code to the repository - why? Because CM is used mostly as library, it should be usable even if 3rd party components will change location / become unsupported.
• CocoaMarkdown can be used via CocoaPods - I added .podspec pointing to the original repo, so it will work after accepting this PR - the latest commit takes care of this.

Finally, I've corrected minor issues in example projects so they are compatible with XCode 7 and Swift 2.X

[indragiek]

@krodak Thank you for this pull request! Adding a podspec for this project is great, but I would like to keep CocoaMarkdown itself using git submodules instead of CocoaPods. Some people will want to integrate the library directly into their Xcode project without using a package manager, and others will want to use other package managers like Carthage and Swift Package Manager (if it eventually supports Objective-C modules). For this reason, it would be preferable if CocoaMarkdown didn't have a build dependency on CocoaPods.

[indragiek]

Podspec was added in #20.