Extract out as many scalactic macros as possible, so that they can be used within Scalactic and ScalaTest builds.

(Snapshots and Requirements has too many dependencies via Prettifier.)

TODO: Scalactic doc output has doc source urls incorrect for scalactic macro files.
Add scalactic macros output to scalatest as well, and exclude from depending on non-existent artifact, per http://grokbase.com/t/gg/simple-build-tool/133shekp07/sbt-avoid-dependence-in-a-macro-based-project
Fix generating source links in api documentation.  This involved:
  * Removing macro classes from doc compile classpath
  * Copying macro sources in docsrc, with corresponding directory structure
  * Adding -Ymacro-no-expand to doc generation scalac options.
  * Refactoring genDocSources to allow multiple srcDirs
  * Changing docSourceUrl to top-level of GitHub tree
  * Changing genscalactic source generation to use sourceDirectory instead of sourceManaged (so that matches on src/ prefix)
Avoid using toPath for Java 6 compatibility.
Output information about directories files copied in GenScalactic, to help debugging.
No longer copy scalautils directory (removed in earlier commit in master).
Remove reference to scalautils in test.
Bumped version to 3.0.0-SNAP4 to test the extra-macro refactor o a deployment to sonatype.