Fix jdbc:postgresql url format to use 'user' instead 'username'.

When using a jdbc:postgresql url valid according to the current ParserURL
the jdbc driver (used by the evolutions plugin) cannot connect to the db
as it's missing the user connection parameter.

According to the postgresql jdbc driver documentation the correct parameter
is 'user' instead of the previously used 'username' - this shouldn't have worked
for anyone.
See: http://jdbc.postgresql.org/documentation/91/connect.html#connection-parameters
Change README to mention 'user' instead of 'username.
Merge pull request #35 from magro/fix-jdbc-postgresql-url-format

Fix jdbc:postgresql url format to use 'user' instead 'username'.