tools/pipbot: Support quietly executing commands

This allows us to invoke shell commands silently, piping output to
/dev/null.

Issue #184.
tools/pipbot: Add a perform_action() function

This is a wrapper around run() which executes a shell command, but
accepts a human-readable string explanation for the for action that is
being performed, and prints this, rather than echoing the command.

Issue #184.
tools/pipbot: Use perform_action() in build

This silences normal execution output of the build command.

Issue #184.
tools/pipbot: Use perform_action() in deploy

This silences normal execution output of the deploy command.

Issue #184.
tools/pipbot: Use perform_action() in undeploy

This silences normal execution output of the build command.

Issue #184.
Merge branch 'feature/184'