Added alias current_orders to open_orders

Given that the documentation specifies that the current_orders returns the 'pending' orders the user currently has on CEX.IO, it makes sense to add this alias, in case anyone is already using open_orders the alias will not break his code when updated.

Additionaly, added indentation to the class inside the module (ruby convention => classes inside modules should be indented, mostly for readability)
Merge pull request #5 from fhur/patch-1

Added alias current_orders to open_orders