Discovering main classes

I added a feature to sbt-pack (for personal needs), and was wondering if you could be interested in it.

I added a `packMainDiscovered` task, that uses sbt's task `discoveredMainClasses` (which basically gives a list of the main classes in the current project) to guess a possible value for `packMain`. It can be used this way to define `packMain`:

    packMain := packMainDiscovered.value

If your project only main classes are `com.myproject.FirstProg` and `com.myproject.OtherProg`, it gives `packMain` the following value:
    Map("first-prog" -> "com.myproject.FirstProg", "other-prog" -> "com.myproject.OtherProg")

(It hyphenizes the names the main classes to get the scripts' names.)

I changed the tests in place to illustrate it (this make the diff easier to read, although I think the right way to go should be to keep the existing tests as is, copy them, and change the copied tests).

Let me know if you have any question or remark.