Increase salt length
Implement UserInterface in User class
Encode password in insertion and update methods
Use key constants for our own field description
Code refactor and style following
Make password invisible
Add possibility to distinguish create and edit options

Add predicat function to determine whether we are creating or updating
object and special field in description of entity fields describing when
should apply this option. It is needed, for example, to edit password
field. When new user gets created, password is required. In editing case,
password can be blank.
Make password not required during editing user entity
Merge pull request #34 from MultiTeemer/master

Close #30 and add some editing feature