External download links handling

Hello again,

I added a new feature: now if a page (such as https://pypi.python.org/simple/nose/ or https://pypi.python.org/simple/py-bcrypt/ or https://pypi.python.org/simple/PIL/) does not have direct links to package files, the proxy will make some voodoo to make it downloadable anyway (via a special attribute ```?remote=<remote_url>```). See the code for details :-)

The system should now work with all packages :+1: 

Signed-off-by: robin-jarry <robin@jarry.cc>