gpg: add initial bare-bones wrapper

Library/Homebrew/gpg.rb

+require "utils"
+
+class Gpg
+  # Should ideally be using `GPGRequirement.new.gpg2`, etc to get path here but
+  # calling that directly leads to:
+  # requirement.rb:139:in `which_all': uninitialized constant Requirement::ORIGINAL_PATHS (NameError)
+  # when i.e. including the gpg syntax in wget. Not problematic if not used by formula code.
+  # For now, the path determination blob of code has been semi-modified for here.
+  # Look into this more.
+  def self.gpg
+    which("gpg") do |gpg|
+      gpg_short_version = Utils.popen_read(gpg, "--version")[/\d\.\d/, 0]
+      next unless gpg_short_version
+      Version.create(gpg_short_version.to_s) == Version.create("2.0")
+    end
+  end
+
+  def self.gpg2
+    which("gpg2") do |gpg2|
+      gpg2_short_version = Utils.popen_read(gpg2, "--version")[/\d\.\d/, 0]
+      next unless gpg2_short_version
+      Version.create(gpg2_short_version.to_s) == Version.create("2.0")
+    end
+  end
+
+  GPG_EXECUTABLE = gpg2 || gpg
+
+  def self.available?
+    File.exist?(GPG_EXECUTABLE.to_s) && File.executable?(GPG_EXECUTABLE)
+  end
+
+  def self.create_test_key(path)
+    odie "No GPG present to test against!" unless available?
+
+    (path/"batch.gpg").write <<-EOS.undent
+      Key-Type: RSA
+      Key-Length: 2048
+      Subkey-Type: RSA
+      Subkey-Length: 2048
+      Name-Real: Testing
+      Name-Email: testing@foo.bar
+      Expire-Date: 1d
+      %commit
+    EOS
+    system GPG_EXECUTABLE, "--batch", "--gen-key", "batch.gpg"
+  end
+end