refactor(sprocket/stream): just exposed @_dispatchStream is enough

* the _internalStreams object should be kept inside closure
* SprocketStream::_transform writes to _dispatchStream and will write back
feat(sprocket/ext/less): add LESS preprocessor support
docs(examples): update dependencies for bootstrap LESS
feat(package.json): add less extension support
docs(examples): add admin.less as compile example
chore(release): 157d11ca4e by Makefile