Make this compatible with Windows `kindlegen.exe' binary

Modify extconf.rb to change target based on platform because Windows binary is
`kindlegen.exe' and not `kindlegen'. Include simple tests to see if `curl' and
`unzip' exist in Windows PATH. Make code cleaner.

Modify kindlegen.rb to get command by joining binary directory and "kindlegen".
The previous scheme fails to return the correct command if the binary is
`kindlegen.exe'

.gitignore

@@ -1,4 +1,5 @@
 *.gem
+*.swp
 /pkg/*
 /.bundle
 /vendor/bundle

ext/kindlegen/extconf.rb

@@ -4,34 +4,51 @@
 
 require 'rbconfig'
 File::open('Makefile', 'w') do |w|
-  tarball = case RbConfig::CONFIG['host_os']
+  case RbConfig::CONFIG['host_os']
   when /mac|darwin/i
     unzip = 'unzip'
-	 "KindleGen_Mac_i386_v2_9.zip"
+    tarball = 'KindleGen_Mac_i386_v2_9.zip'
+    target = 'kindlegen'
   when /linux|cygwin/i
     unzip = 'tar -zx --no-same-owner -f'
-	 "kindlegen_linux_2.6_i386_v2_9.tar.gz"
+    tarball = 'kindlegen_linux_2.6_i386_v2_9.tar.gz'
+    target = 'kindlegen'
   when /mingw32/i
     unzip = 'unzip'
-    "kindlegen_win32_v2_9.zip"
+    # Abort if either `unzip' or `curl' if not found
+    `where #{unzip}`
+    unless ($?.success?)
+      STDERR.puts "The program `unzip' not found. Aborting."
+      exit(1)
+    end
+    `where curl`
+    unless ($?.success?)
+      STDERR.puts "The program `curl` not found. Aborting."
+      exit(1)
+    end 
+    tarball = 'kindlegen_win32_v2_9.zip'
+    target = 'kindlegen.exe'
   else
     STDERR.puts "Host OS unsupported!"
     exit(1)
   end
 
+  bindir = File.join(File.expand_path('../../..', __FILE__), "bin")
+
   config = RbConfig::CONFIG.merge({
     "unzip" => unzip,
-    "tarball" => tarball
+    "tarball" => tarball,
+    "target" => target,
+    "bindir" => bindir,
   })
 
-  bindir = File.join(File.expand_path('../../..', __FILE__), "bin")
-  w.puts RbConfig.expand(DATA.read, config.merge('bindir' => bindir))
+  w.puts RbConfig.expand(DATA.read, config)
 end
 
 
 __END__
 AMAZON = http://kindlegen.s3.amazonaws.com
-TARGET = kindlegen
+TARGET = $(target)
 BINDIR = $(bindir)
 TARBALL = $(tarball)
 CURL = curl

lib/kindlegen.rb

@@ -5,16 +5,12 @@
 module Kindlegen
   Root = Pathname.new(File.expand_path('../..', __FILE__))
   Bin  = Root.join('bin')
-  Executables = Bin.children.inject({}) { |h, p|
-    h[p.basename.to_s.to_sym] = p.to_s
-    h
-  }
 
   #
   # Getting command path of kindlegen.
   #
   def self.command
-    Executables[:kindlegen]
+    Bin.join('kindlegen')
   end
 
   #