Revert "Load global plugins in their own class loader and replace the base loader with that.  Fixes #272."

This reverts commit 8cb7e230110e846b577539e61757e815f083294e.

Conflicts:
	main/src/main/scala/sbt/Load.scala
Re-add scripted test
build projects' transitiveUpdate to depend on globalPluginUpdate
Make GlobalPluginData preserve compatibility wrt .unapply
Merge pull request #1352 from dansanduleac/issues/1084

Don't inject global plugins' classpath into that of builds, and autotrigger build.update if GP.updated