Changed grant type on password auth to 'password'
Changed grant type on password auth to 'password'

Podio authentication function authenticate_with_password incorrectly used 'username' as its grant type, resulting in a (fatal) auth error. Changed to 'password' to fix.
daniel-sc/podio-php,enragedlemon/podio-php,podio/podio-php,NickIvanter/podio-php