Supports platform Xamarin.iOS in the Nuget package

Adding the support of the Xamarin.iOS from the @nashspence pull request #4 .

Merge updates from upstream.

Some changes are commited on the original project, this release apply these changes.

Virtual File System implementation

Create a Virtual File System structure for using SharpZipLib whith file names like original Library.

Read readme.md for more information about using VFS.

Initial PCL release.

This is the initial PCL version.

We maintain the same version number with the original #ziplib for the moment.