clean import and connection to mongokit initilization

1) the common __init__ instanciate a connection with database
   all module nedde to use the data base need to use this connection
   from mobyle.common import connection

2) the registration in mongokit is done using decorator
   connection.regiter at the top of each class need to be registered

3) cleaning all imports according 1) and 2)
   use relative imports when it's possible
  cleaning redondant import, unused imports