API for distributed cache

[johnynek]

Make this easier to use from scalding.

[avibryant]

Suggested API:

class Cache[T](path : String) {
  def load : T
}

object Cache {
  def apply[T](value : T) = new Cache[T](serializeAndPlaceInDistCache(value))
}

Usage:

val cachedData = Cache(loadLocalData)
lazy val data = cachedData.load

[ktoso]

Was thinking of picking this up, but it turns our we have it already:
https://github.com/twitter/scalding/blob/develop/scalding-core/src/main/scala/com/twitter/scalding/filecache/DistributedCacheFile.scala added in 7fd4b15

A nice to have here would be to expose the add__ToClasspath methods, but in general the existing implementation by @slyphon seems to resolve this task.

[johnynek]

yes, closed by #450

@ktoso the classpath stuff would be great to have.