Add :shell deploy strategy

When :shell is specified as the value of :deploy-via, lein release
will expect the value of the :shell key in the :lein-release config to
be a vector of shell command args, like:

:lein-release {:deploy-via :shell
               :shell ["lein" "deploy-uberjar"]}
Add documentation to README
Merge pull request #1 from utahstreetlabs/shell

Add :shell deploy strategy