azure active directory authentication support

[alexturkin]

Problem

in order to be able get data from Azure Insights REST we need to obtain authorization token from Azure Active Directory

Solution

implement functionality to work with Azure ID via two ways:

• user in AD: username & password
• AD application: client id and client secret

[alexturkin]

sorry, my intention was to submit it on review to https://github.com/alertlogic/erlazure/ fork first