Check for network connectivity before POSTing.
Only expose a `setNetworkConnected` mutator fn.
Always return true for isNetworkConnected in Java.
KeenNetworkStatusHandler interface.

This replaces the Builder.isNetworkConnected method with a
KeenNetworkStatusManager interface, a default implementation
(AlwaysConnectedNetworkStatusManager), and an Android-specific implementation.
Long lived references to the builder object are remove from the KeenClient
object.
Whitespace cleanup.
handleFailure on no network.
Revisions from @geeber.
Two more @since updates.
Merge remote-tracking branch 'smurthas/reachability' into 2.1.0

* smurthas/reachability:
  Two more @since updates.
  Revisions from @geeber.
  handleFailure on no network.
  Whitespace cleanup.
  KeenNetworkStatusHandler interface.
  Always return true for isNetworkConnected in Java.
  Only expose a `setNetworkConnected` mutator fn.
  Check for network connectivity before POSTing.