Updated grunt package to use rsync instead of copy

[grayside]

The same speedup advantage provided in #245 can be had for the grunt package command by replacing it's grunt-copy implementation with grunt-rsync. Currently, larger Drupal sites such as those based on Atrium blow the default Node callstack, and fail to complete after several minutes. I've tested an rsync implementation that works with no problem.

The downside here is that grunt-rsync is

1. not easily assumed as available on Windows
2. not configuration compatibility with grunt-copy, or more specifically, with the minimatch library which drives the file globbing for copy.

In order to address these issues but still default to fast, I propose we take the following steps:

1. In the event that the new util.canRsync function returns false or grunt package is invoked with a new --copy flag, use copy, otherwise we default to rsync.
2. Document that minimatch's more advanced features should not be used.
3. Possibly adjust the default package configuration to comply if it does not already, in both the GDT tests and generator-gadget.

[grayside]

#255 duplicates this. Both by me, same week. Bah.