Feature/simplify client creation

[Samwel K. (samwelkanda)]

Simplifies the initialization of a graph client from

auth_provider = AzureIdentityAuthenticationProvider(credential, scopes=scopes)
# Initialize a request adapter. Handles the HTTP concerns
request_adapter = GraphRequestAdapter(auth_provider)
# Get a service client
client = GraphServiceClient(request_adapter)

to

client = GraphServiceClient(credentials=credential, scopes=scopes)

closes #180