fixing Path comments

## motivation
The comments for Path are wrong around the ```case class /``` and the ```object /:```.  For inexperienced scala users, /: could be especially hard to use because it is right associative.

## sbt changes
I made three changes to the way that sbt builds the project.  First, the default scalaVersion is specified, so if a user has a weird default scalaVersion, it works anyway.  Initial commands are invoked when the console is loaded for finagle-http, because I think the path dsl will probably be invoked frequently when people use the console.  The scalaVersion is specified in the plugins.sbt too, because it grabs its scalaVersion from a different place than the build.scala.