Add login functionality

Validations for emails presence and format and uniqueness
Validations for username uniqueness and length
Validation for password presence  (which doesnt work)

Was not able to figure out how to do password lenght, since the password
is stored as a hash regardless of length. This actually allows for blank
passwords to be created. I tried making custom validations but I could not
figure out how to get everything working and playing nicely with bcrypt.

Logout button uses a delete route and only shows up if user is signed in.
Current_user helper is used to show the user his/her username upon signin.

Did not include different access levels.
Errors are displayed if
Add some basic styling

Some nice to haves:
* passowrd length validation
* access levels
* error alerts that disappear upon refresh
* better styling
Add alerts for errors in create user form.