Add gemspec

We used to include this in Causes as a git submodule. But that's not
really convenient to do with Capistrano.

Since this code is touched so infrequently, it makes sense to separate
it from the Causes repo itself into a gem.

Change-Id: Icfe001cf666f3380dcd1e91aac739f89b4045b94
Reviewed-on: http://gerrit.causes.com/44369
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Tested-by: Tom Dooner <tom.dooner@brigade.com>