Bump version to 0.2.0.dev.
Bump copyright year.
Remove Version History section.
Fix timing vulnerability.

A timing vulnerability was discovered in our security token validation
method that could potentially allow an attacker to perform a timing
attack and discover valid security tokens through specially timed and
crafted webhookr requests. Using a secure comparison on the token will
ensure a constant time comparison and avoid such timing vulnerabilities.

Thanks to Tomasz Pewiński for the vulnerability disclosure and patch.
Add a note about the timing attack vulnerability in Webhookr::Service#validate_security_token.
webhookr also works with rails 3.2.
Add J Smith as an author.