Add createCombo method and example. Fix #20.

[tredondo]

Since @maxicus mentioned he used combos all the time, I've added a createCombo method to simply combo creation.

The example has been tested to work beyond creating the combo. For example, getHistoricalData BID_ASK does obtain correct data for the combo.

[maxicus]

@tredondo good example. one thing here is that production apps usually already have conIds and other contract details (to take real time data for example) so that method in api class is questionable.

can you make that example standalone please? that's for sure valuable to have it.