Added CreateIndex methods to SQLiteConnection

Added CreateIndex methods to SQLiteConnection to make using POCO objects easier.

Usage is:
_Connection.CreateTable<MyTable>(CreateFlags.AllImplicit);
_Connection.CreateIndex<MyTable>(c => c.MyProperty);

or _Connection.CreateIndex("MyTable", "MyProperty");
or _Connection.CreateIndex("Idx_Name", "MyTable", "MyProperty");