Added SequentialGuid Generator

Hi!

I am not so happy with the Guids that are generated by SQLite as they are not sequential and might cause performance issues.
Recently, I came across the following article on codeproject: http://www.codeproject.com/Articles/388157/GUIDs-as-fast-primary-keys-under-multiple-database

I adapted the code a little so it also runs on windows phone/store

Also, I chose to follow an approach that causes the mnimum modification of SQLite.cs: "IAutoGuidGenerator"

When it comes to perf: I did a small comparison ANDROID app on my Samsumg Galaxy S4 Mini where I made 30 test runs.
One with Guid.NewGuid and the other one with the SequentialGuidGenerator. 
For 100.000 guids:
Guid.NewGuid() takes ~2.9 seconds on average
Sequential takes ~4.0 seconds on average

This disadvantage in performance is acceptable to me. What do you think?