Changes

Adds non-nullable and unique checks to the Registration.login field
The Registration.login field should be treated as the User.login field. 
As the User.login field is the primary key, the non-nullable and unique 
checks are implicitly added. However, as the Registration.login field is 
not a primary key, this checks must be explicitly added.

Updates the database creation scripts
The letta.full.sql and letta.test.sql have being updated to include the 
table creation queries and some User and Event insertion statements.

Updates the contributing guide
The contributing guide has been updated to include the explanation of 
how to contribute to the "tests" module and how to use the fake smtp 
server.