Enforce valid hostnames (minus uppercase) for app IDs

Marathon’s modified hostnames allow digits, dashes, dots, and lowercase
letters. Normal hostnames allow uppercase letters and are case
insensitive, but that would likely be awkward in Marathon's app naming.
Allow assets to be built from any directory.
Merge pull request #321 from mesosphere/build-assets-from-any-directory

Allow assets to be built from any directory.
Document app ID requirements in REST.md

App IDs must match `/^[a-z0-9.-]+$/` currently, and that should be
documented.

Fixes #314.
Update ID regex to current form in REST docs
Blend .form-control alpha more, 0.4 is too dark
Remove unused wrapping .system-folder class

* Alphabetize attributes for .system-caret
Display locale timestamp for task updatedAt

Using JavaScript’s `Date.prototype.toLocaleString`, the timestamp is
displayed in the user’s preferred locale format without using another
library.

Add `title` attribute so hovering the time gives the ISO timestamp.
Merge pull request #323 from mesosphere/locale-timestamps

Locale timestamps
Adding health checks to the task list UI, WIP
Extending the state machine with no tasks state
Moving key to correct element
Adding default health message
-Reverting state machine extension in TaskList
-Adding StackedViewComponent to handle task detail
-Adding TaskDetailComponent
-Moving controlling functionality from AppModal
to TaskList
Adding more robust health message
Adding more detailed task details
Adding footer only on task list/configuration view
Adding taskViewComponent as controller of the task
list
Handling null results
Moving polling to AppModalComp