Add L-BFGS optimiser

[ben18785]

Scipy has it; Stan has it. I think it would be really handy to have it in ask/tell -- I often want to see how close my optimiser's results are over the course of optimisation, for example.

It's a few lines of code to implement this algorithm, so think it should be easy enough to include. Alister is interested and will (once, I get his Github name) assign him tho this.

This is part of #684.

[MichaelClerx]

In anticipation of this, I'm having a quick look at #930 to get the based infrastructure for sensitivity/gradient based optimisers ready.