Support unboxing of mojo parameters.

Currently, the scala-mojo-support code throws an exception if it encounters a mojo parameter with a primitive type.

This change adds support for unboxing, by simply stripping out the offending type check. I did this because:

* Maven already knows what the correct type is for each parameter, because it is listed in the plugin descriptor.
* Java reflection handles unboxing automatically.
* Java reflection will throw an exception itself, in the unlikely event that Maven hands us a value with the wrong type.

This change also adds handling for Maven passing a `null` value to a parameter with a primitive type. I don't think it will ever do this, but in case it does, scala-mojo-support will now react by resetting the `var` to the default value for its primitive type (0 for integers, floats, and characters, and `false` for Boolean).