Allow circular dependencies

Fixes #692.
Update CHANGELOG
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
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
improve error message
improve error handling
formatting
Merge pull request #710 from conda/binary-replace

Binary replace in has_prefix
update changelog
add release date
Merge pull request #707 from asmeurer/lock

Automatically sleep and retry on a lock error
Merge branch 'master' into requests