pip: successfully cache all the packages

Switch to using curl and regular expressions to generate a local cache
file so that we don't need to hit pypi.python.org every time.
This (obviously) results in a massive speed improvement, especially if
you spawn new shells frequently.

This also makes the autocompletion work for me, it didn't before. (pip
would always time out.)

And, also, for fun: This allows you to explicitly set which pip indexes
to use. Technically the old version of the plugin should have had this
behavior automatically -- without having to do more than configure pip
-- but the install completion never worked for me so this is a net gain
in functionality.

plugins/pip/_pip

@@ -6,8 +6,8 @@
 _pip_all() {
   # we cache the list of packages (originally from the macports plugin)
   if (( ! $+piplist )); then
-      echo -n " (caching package index...)"
-      piplist=($(pip search * | cut -d ' ' -f 1 | tr '[A-Z]' '[a-z]'))
+      zsh-pip-cache-packages
+      piplist=($(cat $ZSH_PIP_CACHE_FILE))
   fi
 }