Added podspec file for Cocoapods integration by lucianocn

All you need to do is tag the current build as version 0.0.1:
`git tag -a 0.0.1 -m "Tag release 0.0.1"`
`git push --tags origin master`

Also the spec can than be submitted to CocoaPods or just be used with:
`pod 'NHLinearShadow', :git => 'https://github.com/nthegedus/NHLinearShadow'`

- *Need to verify `.podspec` file if `s.platform` and `s.requires_arc` are correct.*

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**How to submit `pod` to CocoaPods:**
1) Fork and clone `CocoaPods/Specs`.
2) In a single commit, add a folder for your Pod to the main list, as well as the Spec in the format described in the Creating a Pod Repo section.
3) Run pod spec lint to check for errors.
4) If the linter produces errors or warnings, fix them and go back to step 3. If not, continue on.
5) Make your pull request to the master Specs Repo.