Add a human readable helper method for directory mappings

[ivanfrain]

NOTE: pull request #159 follow up.

Currently, a directory mapping can be done in build.sbt with the following snippet (see #160 ):

mappings in Universal <++= (packageBin in Compile, target ) map { (_, target) =>
        val dir = target / "scala-2.10" / "api"
        (dir.***) pair relativeTo(dir.getParentFile)
} 

The idea behind this code is to put in the generated package an api directory with all its content. The original directory is located in target/scala-2.10/api. The target directory will be located at the pacakge root in the "api" directory.

A great enhancement would be to have a helper method simplifying the above snippet to something more comprehensive for human being, for example :

mappings in Universal ++= allFilesRelativeTo(file(target / "scala-2.10" / "api"))

[muuki88]

I think this one is done.

[belgacea]

@muuki88 Is this feature abandoned?

[nigredo-tori]

@belgacea, I don't know if this was ever implemented in SNP itself, but if you're on SBT 1.0, sbt.Path.directory seems to do exactly this.

[muuki88]

We move some of those helpers to sbt-io itself and they added quite some more :)