Install fails if laking locales, build-essential and python-dev

[fbidu]

Hi,

I'm deploying pyshop on a fresh Ubuntu machine and ran into some problems that may or may not be relevant to be described in the readme file. None of these are problems with pyshop per se but problems with its dependencies

1. If no locales are set, pyvenv fails
   As described here

2. If gcc is not installed, Pyshop's installation fails because cryptacular needs to be built

3. If python-dev is not installed, cryptacular also fails

As I said, I'm not sure if those issues should be addressed in the readme file, but I did run into them while trying to install. I would be glad to contribute and add a note about them.