Add SQLiteConnection.BackupDatabase.

The API signature is copied from the official wrapper.
Version 2.6.0: Add SQLiteConnection.BackupDatabase.
Switch to SafeHandles for mono compatibility.

Mono 3.4 throws a MarshalDirectiveException: "Type System.Runtime.InteropServices.CriticalHandle which is passed to unmanaged code must have a StructLayout attribute." when trying to marshal a CriticalHandle.
Move code into shared projects.
Add MonoTouch support.
Add Mac support.
MonoAndroid support.
Generate debug symbols.
Add nuspec.
Add Windows Store support.
Update to Portable.Data 0.1.1.0.
Ignore .DS_Store files.
Add CLSCompliant attribute.
Fix BackupDatabase test on non-NET45 builds.