Follow the Dispose(bool disposing) pattern described in MSDN

Updated the intptrs to follow Microsoft's guidance. see: http://msdn.microsoft.com/en-us/library/system.idisposable(v=vs.110).aspx

Related, I came upon this since I noticed I was reimplementing Dispose() in my classes for these types, which didn't seem necessary. It also got me thinking that maybe sqlite3_backup_finish/sqlite3_blob_close/sqlite3_close etc. shouldn't be in the public API if the raw intptr types implement IDisposable. I was definitely confused and seems like it could be a source of bugs for users.