bsconfig.json vs package.json for project root location

[bobzhang]

Previously, we rely on the package.json to find project root, since we are going to introduce a build tool and has its own input bsconfig.json (similar to tsconfig.json), it seems we should use bsconfig.json to locate the project root.
This may make sense for us to use opam in the future

[chenglou]

bsconfig!

[bobzhang]

@chenglou lets go with bsconfig.json, so package.json is only needed when you publish it

[bobzhang]

done in master