Use ExponentialUtilities.exponential! instead of exp and add method as an option in solvers that currently use exp and expv

[howsiyu]

As per title. My main motive for this is that exp doesn't support AD (JuliaDiff/ForwardDiff.jl#174). This shouldn't affect existing code if we set the default method to be ExpMethodNative.

[ChrisRackauckas]

We should just change it to ExpMethodGeneric which should be faster. All of the methods solve to floating point accuracy so there shouldn't be a noticable difference to the user, though it should open up autodiff support and improve performance.