2.0.0-pre27 -- Fixes adzerk/boot-cljs#5 -- add guard to watch task

- The watch task should have been catching exceptions that are raised
  during the build. Added a guard and print stack trace.
Merge branch 'master' into tempdirs

Conflicts:
	boot/core/src/boot/task/built_in.clj
Add cp! fileset method, improve fileset immutability

- The cp! protocol method replaces a tmpfile in the fileset with the
  contents of the given java.io.File object.

- Blob storage files now have writable permissions off to prevent blobs
  from getting out of sync with their content hashes.