Version checking done in background.

Background version checking done with AsyncTask to improve performance on slow/faulty network connections
UpdateChecker now extends Observable, notifies observers when background version check is completed.
Updated jar file with changes from previous commit
Rolled version number to 2.1 and updated changelog
Storage state checked before downloading.  Consolidated download code into helper method.
APK is downloaded to app-specific external storage directory.

Implemented in the form of a helper method to get the correct directory

Uses hardcoded storage path (/Android/data/<PACKAGE>/files) on API < 8
This has NOT been tested

Uses getExternalFilesDir() on API > 8
This has been tested on gingerbread

Also changed the filename to a constant.
Logged exceptions pass the Exception object to Log.? as well.
Updated changelog
jar file updated to latest version
Updated readme
Added build.xml
Merge pull request #1 from Kiirani/master

getting pull request from Kiirani