Implement non-blocked sampling for array based samplers.

Before, all our samplers operated in the joint space (i.e. blocked) by default. This often leads to bad convergence for metropolis or slice.

This adds functionality to set `blocked=False` when instantiating any sampler which will instead create a step-method for each RV and append them to a `CompoundStep`.

Samplers like metropolis and slice default to not being blocked (as was the case in pymc2 and is what people assume). HMC and NUTS however default to being blocked as they are specifically designed to operate in the joint space.

Unfortunately because of all the configurations `ArrayStep` could be called there's quite some code to determine the configuration it was called in but it's fully backwards compatible that way.

This supersedes https://github.com/pymc-devs/pymc/pull/450 which took a different approach.
@jsalvatier Do you have another idea for implementing this that has the same UX? I don't think it's ideal but personally can't think of a better way.