Change CorePlot source from Mercurial to zip file

The Mercurial repository of CorePlot is huge and Mercurial doesn't support shallow clones. The previous podspec would clone a huge part of the CorePlot repository resulting in a download of at least 750MB. I've changed the podspec to target the zip file of only 8MB. Resolves issue: https://github.com/CocoaPods/Specs/issues/2892

Additional info on the pre_install step:
- first it changes the current directory to the CorePlot pod folder
- the zip file has a deeper structure than necessary, we can't use the flatten option because it has multiple root folders, so I look for the framework folder and bring it to the top
- after that we have to generate the header file for CorePlotProbes based on CorePlotProbes.d

The ```File.exists?('framework')``` makes sure that these steps occur only once. ```Pre_install``` is also called for pod updates which would cause an error with both the move and dtrace command.

/cc @alloy