Convenient aliases for the SBT build

The idea here is to avoid the need for switching
between the SBT shell and Bash.

  - Add aliases for partest, scala{,c,p,doc}
  - Change working directory for these forked processes
    to the root project base directory, rather than the
    subprojects base directory. This lets us use relative
    paths to files in a more familar way.
  - Don't log the output of forked processes with the
    `[info] ` prefix, rather pass it through directly
    to stdout.

Demo:

```
> partest --terse test/files/pos/t6231.scala
[info] Packaging /Users/jason/code/scala2/build-sbt/pack/lib/scala-partest-javaagent.jar ...
[info] Done packaging.
Selected 1 tests drawn from specified tests

.

[info] Passed: Total 1, Failed 0, Errors 0, Passed 1
[success] Total time: 3 s, completed 05/02/2016 12:44:19 PM
> scala sandbox/test.scala
[info] Running scala.tools.nsc.MainGenericRunner -usejavacp sandbox/test.scala
Hello, World!
[success] Total time: 4 s, completed 05/02/2016 12:45:08 PM
> scalac sandbox/test.scala
[info] Running scala.tools.nsc.Main -usejavacp sandbox/test.scala
[success] Total time: 3 s, completed 05/02/2016 12:45:15 PM
> scala Test
[info] Running scala.tools.nsc.MainGenericRunner -usejavacp Test
Hello, World!
[success] Total time: 1 s, completed 05/02/2016 12:45:20 PM
> scala
[info] Running scala.tools.nsc.MainGenericRunner -usejavacp
Welcome to Scala 2.11.8-20160204-090931-42525ec (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_71).
Type in expressions for evaluation. Or try :help.

scala> 1.to
to   toBinaryString   toByte   toChar   toDegrees   toDouble   toFloat   toHexString   toInt   toLong   toOctalString   toRadians   toShort   toString

scala> 1.toString
res0: String = 1

scala> :quit
[success] Total time: 8 s, completed 05/02/2016 12:45:48 PM
>
```

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