Tweak build script to only set version number if we're in a git repository

Add readme for GHUnit

@@ -3,7 +3,7 @@
 # This script sets a version number for ASIHTTPRequest based on the last commit, and is run when you build one of the targets in the Xcode projects that come with ASIHTTPRequest
 # It only really needs to run on my computer, not on yours... :)
 require 'find'
-if (File.exists?('/usr/local/bin/git'))
+if (File.exists?('.git') && File.directory?('.git') && File.exists?('/usr/local/bin/git'))
 	newversion = `/usr/local/bin/git describe --tags`.match(/(v([0-9]+)(\.([0-9]+)){1,}-([0-9]+))/).to_s.gsub(/[0-9]+$/){|commit| (commit.to_i + 1).to_s}+Time.now.strftime(" %Y-%m-%d")
 	buffer = File.new('Classes/ASIHTTPRequest.m','r').read
 	if !buffer.match(/#{Regexp.quote(newversion)}/)

+This folder is where ASIHTTPRequest looks for GHUnit frameworks.
+
+When you run one of the test targets, a build script will attempt to download a pre-compiled framework and install it here, if one does not already exist. If you would prefer to build GHUnit yourself, simply grab a copy from https://github.com/gabriel/gh-unit, and drop your built framework in this folder.
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