Add library app (fix #7 #10)

Ref: #7 #10

This only adds the basic logic of the app. More to come in other PRs :)
I missed your point about PostgreSQL.

Two things:
- we are targeting sqlite only at this point (but of course I don't read in the future ;) )
- I read "Null values are not considered equal." from [psql doc](http://www.postgresql.org/docs/9.3/static/indexes-unique.html)

Am I missing a point?