Release RBTools 0.5.7.
Merge branch 'release-0.5.x'
Bump to 0.5.8 dev.
Merge branch 'release-0.5.x'
Bump to 0.6 dev.
Fix the version number in the release notes.
Merge branch 'release-0.5.x'
Mention cookies filenames in the docs.

A long while back, someone made the extremely reasonable request that the
rbtools docs explain about the cookies files. This is a pretty trivial change.

Fixes bug 2342.