Add an is_connected method on the AbstractStore.

Enpkg should check if its remote is connected to see if we are
connected, instead of having it's own flag.  This makes it easier to
use the Enpkg class programatically, and also brings the API closer to
the AbstractStore in encore.
Merge pull request #118 from punchagan/store-is-connected

Add an is_connected method on the AbstractStore.