Postgresql removed

I decided to switch from Postgresql to in-memory Sqlite as backend database to remove the only external dependency Postput had and to improve performance!
I actually first chose postgresql for convenience since I knew that technology but then I started feeling like it was not the good technology since a call to an external database had to be done every time a file was fetched. Sqlite being in-memory, querying it a lot won't hurt performance, even on production.
Postput can now be launched directlyy, without any other requiremements!