Add setUsername and setUserProperties to $analytics API + registration functions to $analyticsProvider
Register setUsername and setUserProperties for mixpanel people
Use buffers to catch early calls to setUsername and setUserProperties
Add setUserPropertiesOnce to angulartics and to mixpanel
Added setUserPropertiesOnce to public API
Merge branch 'feature-properties' of https://github.com/sampov2/angulartics into sampov2-feature-properties

* 'feature-properties' of https://github.com/sampov2/angulartics:
  Added setUserPropertiesOnce to public API
  Add setUserPropertiesOnce to angulartics and to mixpanel
  Use buffers to catch early calls to setUsername and setUserProperties
  Register setUsername and setUserProperties for mixpanel people
  Add setUsername and setUserProperties to $analytics API + registration functions to $analyticsProvider
Merge branch 'sampov2-feature-properties'

* sampov2-feature-properties:
  Added setUserPropertiesOnce to public API
  Add setUserPropertiesOnce to angulartics and to mixpanel
  Use buffers to catch early calls to setUsername and setUserProperties
  Register setUsername and setUserProperties for mixpanel people
  Add setUsername and setUserProperties to $analytics API + registration functions to $analyticsProvider