Encrypt secrets in database

This changeset addresses issue #26:

- Enlarging secret columns. I'm not sure what's the correct length for those fields, but I picked 132 since that seem to be the minimum length when used with **rijndael** in my dev machine.
- While we're changing schema, I added `name`, `created` and `modified` fields for `clients`
- Extract out the hashing method into `OAuthUtility` class and trying to be BC.  This class would also later be used to implement the oauth-php interface methods.
- Add a new shell to create and list client records. It can display decrypted secrets with --secret.  I think this is quite handy.

**Note:**

> Since this change is backward incompatible, ie changing schema and requiring secrets to be reissued, this 
> should go into a new branch.  I think we should tag a version on master first, then create a separate branch for 
> the version