Annotate functions returning a node-value

Add {:pallet/plan-fn true} metadata to each function that returns a node-value.
Fix plan-when script generation bug

plan-when was generating invalid script, due to unquote not being scoped correctly.
Require clojure 1.4.0

Remove use of slingshot
Add location information to exec-checked-script

In order to improved the utility of the logs, add file and line number to
message string passed to exec-checked-script.
Add function to throw operation phase errors

Adds the pallet.core.api/throw-phase-errors function that will throw
an exception if the passed operation has phase errors.
Adjust script execution logging

Uses the new "#> " status line prefix in stevedore to help make
the logs more searchable.
Update for new source line comments in stevedore