Added recursive management of minion.d and master.d.

This allows users to easily add configuration settings that the formula
doesn't yet provide, without having to modify the existing templates.
Merge pull request #51 from andrew-vant/salt.d

Added recursive management of minion.d and master.d.
Readme note about user-provided configuration templates.
Merge pull request #52 from andrew-vant/salt.d.doc

Readme note about user-provided configuration templates.