(GEP-164) Handle dropped support for operator += in 4.0

Add a preference that controls reporting when the deprecated
+= operator is used. The default setting for this preference
is 'warning'. The preference is hardcoded to error in when
using version 4.0.
Merge pull request #40 from puppetlabs/issue-164

(GEP-164) Handle dropped support for operator += in 4.0