Added file REQUIRE.

REQUIRE lists StatsBase as a dependency for this package.
Dealing better if nonconvergence. Removed warnings.

Now nonlinear_fit is dealing better with non-convergence.
Now, nonlinear_fit returns a boolean stating whether the
function converged and the number of iterations it took to
converge as well as the coefficientes.

Changed 'max' to 'maximum' to remove a warning.