Update create_dev_user

addresses issue described in #259 https://github.com/CartoDB/cartodb/issues/259:

* replaces call to rake cartodb:db:setup with calls to individual rake tasks 
* although cartodb:db:setup relies on "rake:db:create", "rake:db:migrate", and "cartodb:db:create_publicuser", each of which call :environment, environment is not always loaded correctly, sometimes yielding error described in #259 
* Here the script calls each rake task in turn, adding a final call to the existing cartodb:db:create_user.
* Each of these tasks explicitly load :environment; no more error
* tested on clean installs of hosted cartodb on Ubuntu 12.04 64-bit