More user friendly budget specification options

[TheEimer]

I always find it really hard to specify the budget for SH/HB correctly since SMAC wants the trials, while I want epochs, steps, etc. - so I have to literally reverse engineer the scheduler to start SMAC correctly! I know that I could set a timeout, but most ML might not want that. Enabling something like a "total_budget" option for MF would be extremely useful and will make SMAC a lot easier to use for non-AutoML folks.