Go wrapper for libLBFGS

[kho]

Inter-operation is achieved with cgo. The wrapper offers an idiomatic
Go package for objective function minimization via L-BFGS.

Slight behavioral differences from the C library:

• The optimization routine is renamed to Minimize() (to not repeat the
  package name).

• The optimization routine returns the final value of the objective
  function. The C library allows ptr_fx to be NULL when the user does
  not care about the value. In Go this is usually done by assigning _
  to the return value, e.g.

  _, err := lbfgs.Minimize(...)
  

  My question is whether setting ptr_fx to NULL saves any call to
  evaluate(). If that is true, then the design in Go has a
  slightly higher cost when the user does not need the final
  objective and we should revert to the C design.

• All errors returned from C lbfgs() is translated to Go errors
  (except LBFGS_SUCCESS, in which case nil error is returned as is
  idiomatic in Go).

[cynthia]

This patch may need extra work if we land #23.