docs(CONTRIBUTING): add code style section
Merge pull request #613 from unihorn/29

docs(CONTRIBUTING): add code style section
fix(CONTRIBUTING): minor grammar fixes
fix(server/transporter.go) fix the short timeout problem of sending snapshot
Snapshot can be tens of MB. We need to have a minute level timeout for sending snapshot.
We add a separate snapshot http client for snapshot request, which has 120s timeout.
fix(store): corrected CAS and CAD fail cause in response

specifically when both prevIndex and prevValue are provided
Merge pull request #614 from metaflow/fix-611

fix Compare and swap returns incorrect cause
Merge pull request #616 from xiangli-cmu/add_snapshot_post

fix(server/transporter.go) fix the short timeout problem of sending snapshot
Merge pull request #610 from metaflow/master

docs(api) added note for CAS about prevIndex=0 and `cause` meaning in server response