Update tox to use develop and custom pip command

The use_develop option saves time over the sdist/install. More
importantly, install_command allows us to override tox's default
behavior of passing --pre.

Change-Id: Ifb8fc877aa23db38310777912aa6d914ae040178

tox.ini

@@ -1,8 +1,11 @@
 [tox]
-distribute = False
+minversion = 1.6
+skipsdist = True
 envlist = py26,py27,py33,pep8,pypy
 
 [testenv]
+usedevelop = True
+install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          LANG=en_US.UTF-8
          LANGUAGE=en_US:en