fix: #67

Basically os.path.commonprefix fails to compare paths with difference on their casing.

'C:\\aaa\\bbbb\\ccc\\ddd'  fails when comparing commonprefix with 'C:\\Aaa\\Bbb\\Ccc\\Ddd'

Output example of sys.path on Windows 7 64b:

	['',
	 'C:\\bin\\Python27\\lib\\site-packages\\pillow-2.0.0-py2.7-win32.egg',
	 'C:\\bin\\Python27\\lib\\site-packages\\pil-1.1.7-py2.7-win32.egg',
	 'C:\\bin\\Python27\\lib\\site-packages\\setuptools-2.2-py2.7.egg',
	 'C:\\bin\\Python27\\lib\\site-packages\\mechanize-0.2.5-py2.7.egg',
	 'C:\\bin\\Python27\\lib\\site-packages\\lxml-3.3.1-py2.7-win32.egg',
	 'C:\\bin\\Python27\\lib\\site-packages\\distribute-0.6.49-py2.7.egg',
	 'C:\\bin\\Python27\\lib\\site-packages\\sqlalchemy-0.9.3-py2.7-win32.egg',
	 'C:\\bin\\Python27\\DLLs',
	 'C:\\bin\\Python27\\lib',
	 'C:\\bin\\Python27\\lib\\plat-win',
	 'C:\\bin\\Python27\\lib\\lib-tk',
	 'C:\\bin\\Python27',
	 'C:\\bin\\Python27\\lib\\site-packages']

Comparison:

 - Agains't **'c:\\bin\\python27\\'** => []
 - Agains't **'c:\\bin\Python27\\'** => []
 - Agains't **'C:\\bin\\Python27\\'** => ['',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27\\',
	 'C:\\bin\\Python27',
	 'C:\\bin\\Python27\\']

This way you can imagine the impact the missing **.lower()** would make on windows