Merge similar optimization methods in C++

[suntzu86]

BLOCKED ON: #274

Currently I have things like: ComputeOptimalPointToSampleWithRandomStarts and ComputeOptimalPointToSampleViaMultistartGradientDescent

The only difference here is that the second one lets you specify the starting_points for MGD and the first one generates them for you. The first one wraps the second one. That's a lot of duplicated comments and junk for such a teeny tiny difference.

[suntzu86]

This would be better solved by something like #274. Having gigantic argument lists where half of them can be null to engage some default behavior is worse than naming these 'endpoints' separately.