[#61] Automatically download new FLOW versions

* Add support library to handle Notifications
* Trigger ApkUpdateService when the app starts
* Display notification if new version is available
* Download and install new version
[#61] Add helper function to compare dot-decimal notation versions
[#61] Fix regex to match dot-decimal notation
[#61] Use info icon for all notifications
[#61] Add 2 step permission to download and install the APK

* Add new MODE to the service to CHECK, DOWNLOAD or INSTALL
* Provide path to local file within the intents
* TODO: Remove old APKs from sd card
[#61] Update APK update notifications

* Externalize strings
* Use compatibility library notifications
* Assign maximum priority
[#61] Add dialog to notify the user 'in your face'

* Move dialog request to Home Activity
* Use non zero IDs for notifications
* Clean up
[#61] Handle null pointer scenarios

* Check for 'listFiles()' null cases
[#61] Check file ETag to ensure a successful download

* Compare etag and file MD5 checksum
* Use the same requestId in notification's PendingIntent
* Read headers (ETag & Content-Length) in the same connection
[#61] Update javadoc with more details
Merge branch 'feature/apkupdate' into develop