Adding benchmarks

Adding a benchmark and making the `keys()` function optional
using lodash by default.

Even though lodash is winning, I'll leave it in to see if something
can improve it.
Sorting on properties

Adding sorting on property values and sorting on properties in
objects. See tests for examples.
adding property descending test

forgot the property decending test
Merge pull request #9 from doowb/master

Some benchmarks and additional options