Use sbt 0.13-style key definitions

In sbt 0.13.0, a new way of defining keys was added. Rather than supplying the name of the key for the command  line as a String, the name of the variable assigned to is used. This commit causes the keys defined in sbt-cpp.scala to use this new definition style. It also fixes a bug: The keys in the NativeBuild object had what were supposed to be their descriptions supplied as their command-line names.