Implement async cell edit

[AllenFang]

#63

• Support Cell Editing with Promise
• Support Cell Editing with Redux way(A little complicated, I'll keep to figure if we can simply it)

Some things need to change after merging this PR:

1. Refine stateful-layer.js
2. Implement a wrapper for sorting
3. Fix promise hard to test problem

[AllenFang]

@Chun-MingChen

I wanna merge this PR ASAP and also I did some changes about design, I'll create another PR to keep to refactor other parts, thanks

1. I create HOC that was charge for cell editing operation so that in this PR bootstrap-table.js will be more clean
   https://github.com/react-bootstrap-table/react-bootstrap-table2/blob/develop/packages/react-bootstrap-table2/src/cell-edit-wrapper.js

2. cell-edit-wrapper.js will be used in stateful-layer, also I will redefine the role of stateful-layer

3. bootstrap-table will be more stateless after this PR and I already decide not to export bootstrap-table in the index.js and we only export the BootstraTable which always wrapper by stateful-layer

I can explain more details on Line if you have any concern and let me know your idea. thanks.