Show “Loading apps…” until first app poll

“No tasks running” was incorrect since the state is unknown. Show a
temporary loading state until the app list is known.

TODO(ssorallen): AppListComponent and TaskListComponent are now largely
the same. They could use a common component.

Fixes #278.
Merge pull request #298 from mesosphere/loading-apps

Show “Loading apps…” until first app poll
Let ancestors access modal to prevent exception

Let the current call stack clear so ancestor components accessing this
modal can still access it before it is unmounted. Without deferring,
React throws an exception in `ReactMount.findComponentRoot` when the “X”
or “Cancel” button is clicked.
Re-tag 0.5.0 release to get travis changes
Back to 0.5.1-SNAPSHOT
Link to 0.5 release from the README
bug fix: New app doesn't immediately display app list
added 'unknown' until running tasks get updated
review: Moving task running to model
review: Discarding wrong changes
Merge pull request #301 from mesosphere/prevent-modal-exception

Let ancestors access modal to prevent exception
Merge pull request #300 from mesosphere/bug/new-app-not-displaying-immediately

Please review