…rgs are present

Adds three new methods of `Arg` which allow for specifying three new
types of rules.

* `Arg::required_unless`

Allows saying a particular arg is only required if a specific other arg
*isn't* present.

* `Arg::required_unless_all`

Allows saying a particular arg is only required so long as *all* the
following args aren't present

* `Arg::required_unless_one`

Allows saying a particular arg is required unless at least one of the
following args is present.

Can now use the `App::before_help` method to add additional information
that will be displayed prior to the help message. Common uses are
copyright, or license information.

…imes duplicatd in error messages

Closes #492