Generalize box constrained optimization slightly.

[pkofod]

Fixes #89

Trying to clean up the issues, I stumbled upon #89 . This PR fixes the "keyword" issues where "eta" and the preconditioner-keywords were forced into the optimizer constructors. This enables

results = Optim.optimize(objective, x0, l, u, Fminbox(), optimizer = LBFGS)

which will be preconditioned due to the recent refactoring + improvement of preconditioning, but also

results = Optim.optimize(objective, x0, l, u, Fminbox(), optimizer = BFGS)

or even

results = Optim.optimize(objective, x0, l, u, Fminbox(), optimizer = NelderMead)

You might not want to do it, but currently, all the methods in Optim should be passable, although Newton will generate a warning and default to CG.

[codecov-io]

Current coverage is 85.09%

Merging #205 into master will decrease coverage by 0.27%

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