Refactored code and added more functions to windows.psm1

New functions: Add-WindowsUser, Delete-WindowsUser
Refactoring and added juju related functions

Moved every juju related function in juju.psm1 module
Refactored juju.psm1 and utils.psm1
Merge pull request #20 from ionutbalutoiu/master

Code refactoring and added new functions