Add JVP implementation of parameter shift rule

[atiyo]

A first implementation of the parameter shift rule.

In the first instance, we have the following limitations:

• We only allow derivatives of expectations with respects to values. In the future it would be cool to allow derivatives with respect to gates as well.
• Spectral gaps are hard-coded, whereas they could be read from the corresponding primitive in the future.

Closes #22

[jpmoutinho]

Not much to say, LGTM!