added intentional failures to some doxbee-sequential tests

As I was discussing bluebird's extreme performance with other developers, they expressed doubt at whether bluebird could be fast when it has do error-catching as opposed to just passing successful promises around.

So I put together a preliminary modification of the three most important tests to give you a possible suggestion on how to make the test more powerful. Here are the old and the new results, using a 50% failure rate:

Old

results for 10000 parallel executions, 1 ms per I/O op

file                                         time(ms)  memory(MB)
callbacks-baseline.js                     425       34.42
promises-bluebird-generator.js        577       38.68
promises-bluebird.js                      682       38.86

Platform info:
Linux 3.11.0-13-generic x64
Node.JS 0.11.8
V8 3.21.18.3
Intel(R) Core(TM) i5-3337U CPU @ 1.80GHz × 4

New

results for 10000 parallel executions, 1 ms per I/O op

file                                         time(ms)  memory(MB)
promises-bluebird.js                      946       41.25
promises-bluebird-generator.js       1017       34.16
callbacks-baseline.js                    1411       38.69

Platform info:
Linux 3.11.0-13-generic x64
Node.JS 0.11.8
V8 3.21.18.3
Intel(R) Core(TM) i5-3337U CPU @ 1.80GHz × 4

Let me know what you think!