Experimental support for SQLite Attributes

Since this PCL targets .NET4 we can't directly reference SQLite-Net PCL
(it targets .NET45 since 2.4.1). So we declare SQLite Attributes which
allows us to flag our classes with proper SQL attributes.

When used on .NET45 PCLs that properly reference SQLite-NET PCL it
should compile just fine and proper assemblies should kick in. We will
still get a warning at build time though (duplicate declaration).