fix installing when not in source directory, fixes #3

setup.py

@@ -2,8 +2,11 @@
 
 from distutils.core import setup
 import re
+import os.path
 
-src = open('jsonpointer.py').read()
+dirname = os.path.dirname(os.path.abspath(__file__))
+filename = os.path.join(dirname, 'jsonpointer.py')
+src = open(filename).read()
 metadata = dict(re.findall("__([a-z]+)__ = '([^']+)'", src))
 docstrings = re.findall('"""(.*)"""', src)