Adding --once argument to test-packages command

So that meteor fails if there is an error and not waits for file change: https://github.com/meteor/meteor/issues/1189#issuecomment-25072673
Merge pull request #7 from mitar/patch-1

Adding --once argument to test-packages command
Forward mrt exit code to make
Merge pull request #9 from mitar/patch-1

Forward mrt exit code to make
Support specifying which packages should be tested
Merge pull request #10 from mitar/patch-1

Support specifying which packages should be tested
Updated README with environment variable
Fix.
Merge pull request #11 from mitar/patch-1

Updated README with environment variables
Correctly detect when Meteor is ready for testing.

See https://github.com/meteor/meteor/issues/1884 for more details.
Merge pull request #14 from mitar/patch-2

Correctly detect when Meteor is ready for testing
need mrt update to ensure to install dependant packages
Allow running custom script for testing
Merge pull request #21 from mitar/patch-3

Allow running custom script for testing