running the bokehserver with splitjs False and devjs True does not work.

This is because there is an inconsistency with build/release js paths, and
what is on the server.

I think it is easier to standardize on our bokeh.js living inside build/js, or release/js

This commit
- modifies the Gruntfile to build into build/js and release/js
- modifies setup.py to look for files in that directory
- renames JSROOT to BOKEHJSROOT, since the name JSROOT is a bit misleading, as we have things like css in there
- fixes an os.path.join call which had 'static/js' as an arguemnt, since that's not platform agnostic


@bryevdv, @mattpap  what do you think?