Fix user iteration callbacks.

User callbacks got broken at some point due to the extra
layer of copying from Solver::Options to Minimizer::Options.
This copies the user callbacks when initializing
Minimizer::Options from Solver::Options, and adds a test to
this effect.

This also fixes a bug where the state updating callback was
not called before the user callbacks. This also adds a test
to solver_impl_test to ensure the state updating callbacks
work as expected.

Thanks to Luis Alberto Zarrabeitia for the report.

Issue: 46
Change-Id: I2b36415c89dafaa5c84ecaa727a325df122e1092