Add JASidePanels.podspec

Hi,

This PR makes it easier for your end users to reference a fork of JASidePanels from their podfile. It does so by adding a podspec to the JASidePanels repository.

For example:

```ruby
pod 'JASidePanels', :git => 'https://github.com/pierlis/JASidePanels.git', :commit => 'e3e162f51f2517cdf66d871d6bcf657a18199f88'
```

Without the podspec, the `pod` command can not reference a specific commit. And this is not handy, whenever a fork is still a work in progress. This is what is fixed by this PR.

NB: if you accept this PR, you will have to maintain this file, and keep it in sync with the spec published on the http://cocoapods.org website.