Add context support to all marketdata.Get* functions

[nathanhack]

All of the functions in the marketdata should accept a context.Context.

Not having it makes using the functions prone to long delays waiting for it to return before being able to respond to a users requests to close an application.

[cksidharthan]

yeah. maybe we can add a WithContext for the existing functions so that we dont have breaking changes to the existing library functions.