Fixes issue #8

Fixes issue #8. Adds new variable mail_header_privacy, on by default.
Installs postfix-pcre unconditionally, and then copies the pcre file
over and adds the header check to main.cf based on the variable value.
“this header replacement works great, but it logs that the replacement
has been done, which means that you are storing this information,
unless you are anonymizing your logs”
Merge pull request #141 from cji/master

Fixes issue #8