Update application/libraries/paymill.php

#1 correct var name (from $param to $params)
fix input parameters

remove wrong parameter an add the right one
fix config setter 
Merge pull request #2 from bmatschullat/patch-1

#1 correct var name (from $param to $params)
Merge pull request #3 from bmatschullat/patch-2

fix input parameters
Merge pull request #4 from bmatschullat/patch-3

fix config setter 
Update README.md

New contributor, the first, thanks Bennet.