Add default make rule

The default make rule builds the extensions "in-place" so that hg can
be run out of the working directory.

Makefile

@@ -2,6 +2,9 b''
 PYTHON=python
 
 all:
+	$(PYTHON) setup.py build_ext -i
+
+install:
 	@echo "Read the file README for install instructions."
 
 clean: