Default user role should be specified by slug, not nicename

The WordPress docs don't make this particularly clear, but at least on 3.9.x, role assignation will fail silently unless you use the lowercase role slug (i.e. 'contributor' NOT 'Contributor').

http://codex.wordpress.org/Function_Reference/wp_insert_user
Merge pull request #10 from peterhartree/master

Default user role should be specified by slug, not nicename