BUG: Correct derivative for exponential transform.
Merge pull request #1540 from jseabold/fix-1539

BUG: Correct derivative for exponential transform.
Merge pull request #1507 from bashtage/silence-tests

CLN: Silence test output
ENH: Avoid RuntimeWarning when hessian unused
TST: Remove unnecessary test.
Merge pull request #1543 from jseabold/tst-fix-warning

TST: Suppress RuntimeWarning for L-BFGS-B
ENH: Separated warnings from other code into a new file

sm_exceptions.py now holds all custom warnings and errors. ConvergenceWarning
was moved to this new module, and imports were corrected.

Cleaned up tests so that docstrings do not obscure names in verbose mode