Add validation for apt::key's name.

It's great to document requirements in README, but error'ing out whenever
the user messes up is even better IMO.

README

@@ -99,6 +99,7 @@ This module needs:
 
 - the lsb module: git://labs.riseup.net/shared-lsb
 - the common module: git://labs.riseup.net/shared-common
+- the stdlib module: https://forge.puppetlabs.com/puppetlabs/stdlib
 
 By default, on normal hosts, this module sets the configuration option
 DSelect::Clean to 'auto'. On virtual servers, the value is set by default to

manifests/key.pp

@@ -1,4 +1,9 @@
 define apt::key ($source, $ensure = 'present') {
+  validate_re(
+    $name, '\.gpg$',
+    'An apt::key resource name must have the .gpg extension',
+  )
+
   file {
     "/etc/apt/trusted.gpg.d/${name}":
       ensure => $ensure,