zpm: abstract the swift_url construction logic

This moves the construction of the swift_url (for the .zar, which looks
like "swift://AUTH_abcdef.../test_container/myapp.zar") to a seperate
util function--with doctests, for good measure.
zpm: abstract `_prepare_job` from `deploy_project`

This util function adds runtime information to the job template json
(namely, the swift:// path to the application .zar).
test_zpm: add `test__prepare_job`

This is a test to exercise `zpmlib.zpm._prepare_job`.
doc/api.rst: include private members of zpmlib.zpm
Merge pull request #75 from larsbutler/refactor-test-deploy-2

Refactor and test `deploy`, part 2