Enable PTR records for BIND driver (#146)

* WIP

* Enable PTR records in dnsconfig.js, in BIND provider.

* Rename REVERSE() to REV().

* More accurate PTR target checking

* Document REV()

* Fix broken test

docs/_functions/global/REV.md

@@ -0,0 +1,32 @@
+---
+name: REV
+parameters:
+  - address
+---
+
+`REV` returns the reverse lookup domain for an IP network. For example `REV('1.2.3.0/24')` returns `3.2.1.in-addr.arpa.`
+and `REV('2001:db8:302::/48)` returns `2.0.3.0.8.b.d.0.1.0.0.2.ip6.arpa.`. This is used in `D()` functions to create
+reverse DNS (`PTR`) zones.
+
+This is a convenience function. You could specify `D('3.2.1.in-addr.arpa`, ...` if you like to do things manually
+and permit typos to creep in.
+
+The network portion of the IP address (`/24`) must always be specified.
+
+Note that the lower bits are zeroed out automatically. Thus, `REV('1.2.3.4/24') is the same as `REV('1.2.3.0/24')`. This
+may generate warnings or errors in the future.
+
+{% include startExample.html %}
+{% highlight js %}
+D(REV('1.2.3.0/24'), REGISTRAR, DnsProvider(BIND),
+  PTR("1", 'foo.example.com.'),
+  PTR("2", 'bar.example.com.'),
+  PTR("3", 'baz.example.com.'),
+);
+
+D(REV('2001:db8:302::/48'), REGISTRAR, DnsProvider(BIND),
+  PTR("1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0", 'foo.example.com.'),  // 2001:db8:302::1
+);
+
+{%endhighlight%}
+{% include endExample.html %}