Allow a single underscore at the beginning

Fixes https://github.com/phpmd/phpmd/issues/113
Added an "allow-underscore" configuration option

Refs https://github.com/phpmd/phpmd/pull/159#issuecomment-49845419
Added tests
Missing authors added.
Merge branch 'patch-1' of https://github.com/ravage84/phpmd into GH159-ravage84-underscore-names
Merge branch 'GH159-ravage84-underscore-names'
Merge branch 'master' of github.com:phpmd/phpmd