Use requests to fetch the repodata
Use requests to fetch the packages
Merge branch 'master' into requests
Correctly handle 304 when fetching repodata
Correctly stream the pkg url so that the progress bar works
Let requests handle the retries
Use requests in download(), and use that in fetch_pkg()
Make sure requests is installed in Travis
Remove unused import
Automatically sleep and retry on a lock error

This uses basic exponential backoff. I'm not sure if that's really needed, but
it does.

Fixes #670
Get rid of old urllib stuff
Enable proxy support in requests (from .condarc proxies)

I haven't tested it yet, but it should work.
Do the right thing when cannot connect to a channel
add ability to handle new has_prefix files
add defaults to new keyword arguments
add implementation of binary_replace
adding file tests
Py3k fixes
fail if placeholder is too short
add docstring