Allow a min and max to be set for responseTime to more accurately mock the variable response times of an actual server.

I thought allowing responseTime to be a bit random would be a nice way to simulate the randomness in timing of responses from a server. If the responseTime is always the same then multiple ajax requests will always be returned in the same order they are made. Making the responseTime variable will help vary the order in which ajax calls return. This is implemented as a two integer array, the first integer is the min, the second the max.

i.e. $.mockjax({responseTime: [100, 750]});

If this PR is accepted I will submit a followup PR with additions to the README.md for this feature.