dev setup for bokehjs,

@bryevdv  can you look this over and see if it's satisfactory?

The main approach is the addition of a bokehjs_file route, which is used to serve anything that goes in builds, including the compiled bokeh.js which maps to bokehjs/builds if debugjs is true, and maps to bokeh/server/static if debugjs is False.  There is another route for bokehjssrc_file which maps to bokehjs/src.  The idea is that the src route is invalid if debugjs is True.  There is another option, splitjs, which tells the URL templates to use require to load the libs, rather than the build bokeh.js

#### Includes

 - supporting incremental builds
 - coffeescript source maps
 - separate js files, vs a single combined bokehjs file

#### TODO

 - extend this to bokehjs demo/test files
 - think about removing all the static files from bokeh/server/static  If the bokeh server doesn't need any of it's own static resources (as in, all static resources come from bokehs, which seems to make sense)  then instead of creating a separate route for the bokehjs build directory, we could just toggle the static path for flask depending on what mode we're running in.  In this case, the bokehsjsrc_file route is still necessary, since that does not always exist for all use cases.