Rule for nullary methods of Unit type.

Warns when a nullary method's result type is Unit. Based on the `-Xlint` rule with the same name. Its code can be found at https://github.com/scala/scala/blob/2.11.x/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala#L1587

Also fixed some dead links in the wiki from my previous pull requests.