Updates to options and a few small bug fixes

Summary of larger updates:

• I got rid of the logitr.summary() function and instead added the logitr class to all the models and renamed the summary function to summary.logitr(). Now you can just use the standard summary() function to summarize model results.
• I finally fixed the analytic gradient for WTP space MXL models. I tested analytic versus numeric for WTP space and Preference Space MXL models and they are all identical, including variations of using normally and log-normally distributed parameters.
• Added startParBounds as an argument in options.

Smaller updates:

• Changed the summary() function to print more digits in the
  summary table.
• Rounded printing of the elapsed time in the summary table.
• Forced the sigma values in MXL models to be positive using abs(). Negative values for sigma parameters should not be an issue because the standard normal is symmetric.
• Changed the summary of random parameters to show "summary of 10k draws"
• Updated hessian to always use numeric approx for SE calculation since it's faster.
• Made scaleInputs default to TRUE.

Bugs fixed:

• If the prefSpaceModel was a multistart, it was grabbing the correct
  bestModel for the WTP calculations, but not the logLik value. Now it's
  getting the right logLik value too.
• Fixed a bug with the scaling option where it was blowing up to use scaling numbers.