JsonPointer.from_parts should handle the empty path

[The RFC says](http://tools.ietf.org/html/rfc6901#page-5) that the path `""` points at the original document. However, passing `[]` to `from_parts()` results in a path of `"/"` (the same as when one passes `[""]`. This makes the empty list produce a pointer for the empty path.

Author: Alexander Dutton

jsonpointer.py

@@ -283,7 +283,7 @@ def from_parts(cls, parts):
         parts = [str(part) for part in parts]
         parts = [part.replace('~', '~0') for part in parts]
         parts = [part.replace('/', '~1') for part in parts]
-        ptr = cls('/' + '/'.join(parts))
+        ptr = cls(''.join('/' + part for part in parts))
         return ptr