Update SlowRetrievalError

Updating on behalf of "Red Team" of App. Sec.
Undescriptive error found while testing gemsontuf.
Fix #150.
Modified the maximum timestamp.txt size (#bug 159)
Merge pull request #161 from nekt/develop

Modified the maximum timestamp.txt size (fix #159).
Merge remote-tracking branch 'upstream/develop' into develop
Add comment to the open() call in tuf.interposition.updater.py

Add link and explain why 'rb' mode is used rather than 'r'.  Prevent newcomers, or future edits, from mistakenly/accidently opening files in text mode.
Merge branch 'develop' of https://github.com/theupdateframework/tuf into develop
Merge branch 'develop' into consistent-snapshots-specification
Merge remote-tracking branch 'upstream/develop' into develop
Merge branch 'develop' into consistent-snapshots-specification