Updated tests to testthat testing framework.

Changed tests to use the testthat testing framework. All tests pass.
Changed default value for maxtime.

The default value for maxtime was change from 0.0 to -1.0. In some cases
the nloptr exit code was 6 (NLOPT_MAXTIME_REACHED) without any
iterations run. Changing the maxtime to a negative value disables this
termination condition and the error no longer occurs with the default
value.
Updated CHANEGLOG and DESCRIPTION