Error message for missing app or stack

Change-Id: If71d0eb50b0198dce3ed97156f7f7cea18b650c2
Replce ref option with filename option

Existing code always packaged HEAD regardless of ref passed in. Since
creating an artifact involves running assets:precompile, HEAD will
always be used as the package name.

Because it may be beneficial to have the ability to create the artifact
under a custom filename, that has been added as an option instead.

Change-Id: I1bc99ab23b892cfff5b0c414c5184a56e29c1374
add permissions information to README

Change-Id: I3d2e66d58c3372a2c46bf9ee2ed98a9f63f862e6
Use PACKAGE_FILENAME environment variable

As an alternative to passing in filename as an argument. This is more
convenient as the filename is used in all three steps of the build
process.

Change-Id: I16caed7f1b46ef3b529b36423030089fc1b0221b
Merge pull request #2 from freerunningtech/filename

Add ability to specify custom filename