Update comments about thread safety
Fix modified (accidently) variable name in keystore.py
Merge branch 'pbkdf2-iterations-vary', remote-tracking branch 'origin' into log
Merge branch 'pbkdf2-iterations-vary', remote-tracking branch 'origin' into integration-tests-python3
Add LOGLEVEL_SCHEMA to tuf.formats

log.py previously checked the format of log level arguments with LENGTH_SCHEMA.  LOGLEVEL_SCHEMA might be clearer.
Update comments and schema type checked

Expand the comments on 'logging.Formatter.converter' to discuss the converter attribute.  Modify LENGTH_SCHEMA to LOGLEVEL_SCHEMA.
Update README.md
Update AUTHORS.txt
Update the valid expiration data
Merge pull request #123 from theupdateframework/log

log.py updates
log.py changes reviewed by both Trishank and Monzur.
Merge branch 'develop' of https://github.com/theupdateframework/tuf into integration-tests-python3