Upgraded login methods to latest API version

[anshu93]

• Changed the login methods to reflect the latest login API payload which has the following format:

'grant_type': 'password', 'scope': 'internal', 'client_id': self.client_id, 'expires_in': 86400, 'device_token': device_token, 'username': self.username, 'password': self.password, [OPTIONAL] 'mfa_code': mfa_code

• Added ability for user to input the device_token through the command line
• Moved the login process to a different module for ease of re-use between the stocks and options downloaders

[joshfraser]

Looks good to me. Thanks for your work on this.