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
Make clojars-url configurable

For people that run their own internal instance of clojars (e.g.
an enterprise repository).

This change was cherry-picked from:

mantree/lein-release@93e663febf9f15ee24433567c2dd314373399878
Merge pull request #5 from joelittlejohn/clojars-url

Make Clojars URL configurable
Merge pull request #1 from utahstreetlabs/shell

Add :shell deploy strategy
note updates
lein-release plugin: preparing 1.0.4 release
lein-release plugin: bumped version from 1.0.4 to 1.0.5-SNAPSHOT for next developemnt cycle