Updated `module/gpg': new functions, cleanup

New internal functions:

    :gpg:kid (key id)
    :gpg:list
Updated `module/util': doc, update

Doc:

    Fixed the usage docstring for `util:secret()'

Update:

    `g_GPG_KEY_PATH' is no more, and site will look for a GPG key
    based on the active profile instead.  No more guess work.
Unit tests for `module/util': nima/site#4
Split `module/vault' from `module/util'

The `vault' module is now the dedicated secret management module.

This new module has also been enabled in `libsite.sh'.
Simplified `share/module/template': copy/paste

The new template is better suited to generating new modules or
updating existing ones; copy/past and amend (rather than try to
teach).
Updated `unit/static-gpg.sh': fix, scaffolding

Renamed `PROFILE' to `SITE_PROFILE' which was overlooked in the last
overhaul commit of `libsite.sh'.

Added setUp/tearDown scaffolding functions for the gpg module.
Updated `module/unit': per-module setUp/tearDown
Updated `.travis.yml'

Install from `share/unit/apt.conf' to prevent user-interactivity.

Install from `share/unit/rng-tools' to speed up RNG; this doesn't
currently work in the Travis CI environment for some reason.
Updated `README.md': Travis CI builds added