Firebird support added

- I have create DbUp.Firebird as discussed with Jake Ginnivan.
- I have tested it with the FirebirdSampleApplication I made. Works fine.
- In addition to the SchemaVersions Table I had to make a SEQUENCE and a Trigger since that is how you implement an Id Column in Firebird.
- I Have used ; as seperator between commands in the script files - could have been anything
- I have changed build.proj and created a new nuspec file - by copying the Postgres stuff (these are not tested).
- Only thing you might want that I have not included is some unittests. The Postgres stuff is not unittested either. Tell me if you want something tested. 

BTW I apparently have 2 identies on GitHub - Jeern and Jesper Niedermann - I was not aware of that. Do not let it confuse you. I am just one person :)