Added PNChart.Framework

Hi,

I have added a framework target to simplify the installation, using the git submobule procedure described below:

1. Add PNChart-Swift as a [submodule](http://git-scm.com/docs/git-submodule) by opening the Terminal, `cd`-ing into your top-level project directory, and entering the command `git submodule add https://github.com/kevinzhow/PNChart-Swift`
2. Open the `PNChart-Swift` folder, and drag `PNChart-Swift.xcodeproj` into the file navigator of your app project.
3. In Xcode, navigate to the target configuration window by clicking on the blue project icon, and selecting the application target under the "Targets" heading in the sidebar.
4. Ensure that the deployment target of PNChart.framework matches that of the application target.
5. In the tab bar at the top of that window, open the "Build Phases" panel.
6. Expand the "Target Dependencies" group, and add `PNChart.framework`.
7. Click on the `+` button at the top left of the panel and select "New Copy Files Phase". Rename this new phase to "Copy Frameworks", set the "Destination" to "Frameworks", and add `PNChart.framework`.

Here all the changes: 

1. Added the Framework Target
2. Added all the compiled sourced in the Framework
3. Changed the visibility to public for framework use for variables and necessary functions

If you prefer you can delete the PNChart-Swift target because is not necessary, but for now I preferred to keep it.