OTWO-2996 Default to EPOCH time when the git parser don't find a date

Currently, it defaults to Time.now.utc which is not reasonably fair.
Also git commands like git log/show defaults to epoch time.
OTWO-2996 Use strict time format check using Time.rfc2822 to handle bogus dates
OTWO-2996 Rename parse_commit_date to parse_date as it can be used to parse author/committer date
Merge pull request #11 from nnsathish/master

OTWO-2996 Default to EPOCH time when the git parser don't find a date