Auto-generating `Makefile` timezone targets.

Updated the `Makefile` to auto-generate timezone data module target
names. The `util/tz2deps.js` program extracts the zone names from one or
more IANA timezone database file and outputs a single `Makefile`
variable definition. This is redirected to a `build/Makefile.deps` file
that is included in the `Makefile`. The `Makefile` then defines the
timezone modules names using the zone names.

GNU `make` will build and rebuild `build/Makefile.deps` when it is out
of date. See the **The GNU `make include` Directive** section of
[Advanced Auto-Dependency
Generation](http://make.paulandlesley.org/autodep.html) for details.
Merge branch `directory` into `travis-ci`.