Update Readme.MD

Replaced all "api" in "var api" and "api." with "cexapi" for better consistency.
Replaced "current_orders" with "open_orders", works with current api.
Update cexapi.js

Renamed "current_orders"  to "open_orders" as current api requests it.
Update LICENSE

Info about contribution added
Merge pull request #3 from coinhelper/master

Replaced "current_orders" with "open_orders"