set-eh: added commands to set event handlers in the manifest

[cgonyeo]

Fixes #77.

[jonboulle]

Hrm, I wonder if there's something we can do here to future-proof appc/spec#58 / rkt/rkt#1495

[cgonyeo]

I imagine if the spec gets changed that more fields would just be added to the event handler object. The tweak that would happen here to accommodate those could just be requiring more arguments get passed in to the command, so something like:

acbuild set-eh pre-start root:root /root/setup-stuff.sh

Changes like that would be fairly easy with the current implementation. If we want to allow for more complex changes (like changing the uid for an existing event handler) another option would be to add subcommands onto pre-fetch and post-stop, but those changes would require more rewriting of this.

[jonboulle]

OK. I guess let's just roll forward and we can cross that bridge. LGTM.