Don't require :role_cname argument for rolify options
Don't require :role_cname argument for rolify options

Related to issue #91.

Defaulting :role_cname to 'Role' with a reverse merge instead of
using it as a default argument allows for more flexible usage
when passing other options into the +rolify+.

e.g.

    rolify :after_add => :after_add_callback

would be allowed now, but before, such a call would require

    rolify :role_cname => 'Role', :after_add => :after_add_callback

RolifyCommunity/rolify,jfilip/rolify,cwingate/rolify,WebEnrich/rolify,gohuygo/rolify,sopheak-se/rolify,jayshreeanand/rolify