Benchmark pacta

Added benchmark for [pacta](https://github.com/mudge/pacta).

I updated locally `latest.md` but did not push it in case you wanted to re-run bench by yourself. Tell me if you want it included in this PR :)

```md
    $ npm ls kew q when rsvp async bluebird deferred liar pacta
    async-compare@0.1.0 c:\Users\Petka Antonov\bluebird\benchmark\async-compare
    ├── async@0.2.10
    ├── deferred@0.6.8
    ├── kew@0.3.4
    ├── liar@0.6.0
    ├── q@1.0.1
    ├── rsvp@3.0.6
    ├── when@3.0.1
    └── pacta@0.5.0

bench doxbee

    results for 10000 parallel executions, 1 ms per I/O op

    file                                 time(ms)  memory(MB)
    callbacks-baseline.js                     391       38.51
    promises-lvivski-davy.js                  900       96.64
    callbacks-caolan-async-waterfall.js      1088       77.33
    promises-dfilatov-vow.js                 1149      144.75
    promises-cujojs-when.js                  1274      109.84
    promises-bluebird-generator.js           1728      108.79
    promises-obvious-kew.js                  1793      173.63
    promises-bluebird.js                     1893      104.67
    promises-tildeio-rsvp.js                 2031      140.68
    promises-pacta.js                        3278      228.02
    promises-medikoo-deferred.js             3567      310.66
    promises-calvinmetcalf-liar.js           7559      290.61
    promises-kriskowal-q.js                 30686      682.18

bench doxbee-errors

    results for 10000 parallel executions, 1 ms per I/O op
    Likelihood of rejection: 0.1

    file                                 time(ms)  memory(MB)
    callbacks-baseline.js                     452       39.51
    promises-lvivski-davy.js                  646       80.30
    promises-dfilatov-vow.js                 1049      156.54
    callbacks-caolan-async-waterfall.js      1096       77.30
    promises-cujojs-when.js                  1234      124.48
    promises-obvious-kew.js                  1698      130.59
    promises-bluebird-generator.js           1746      108.46
    promises-tildeio-rsvp.js                 1959      175.67
    promises-pacta.js                        3098      344.02
    promises-bluebird.js                     5599      115.23
    promises-calvinmetcalf-liar.js           7728      299.30
    promises-kriskowal-q.js                 34454      853.67
    promises-medikoo-deferred.js              OOM         OOM

bench parallel (`--p 25`)

    results for 10000 parallel executions, 1 ms per I/O op

    file                                time(ms)  memory(MB)
    callbacks-baseline.js                    565       46.60
    promises-lvivski-davy.js                1907      255.38
    promises-dfilatov-vow.js                2488      376.80
    callbacks-caolan-async-parallel.js      3900      216.58
    promises-bluebird.js                    4645      315.28
    promises-bluebird-generator.js          4923      335.82
    promises-obvious-kew.js                 6069      720.57
    promises-tildeio-rsvp.js                7155      908.62
    promises-medikoo-deferred.js            7333      573.68
    promises-calvinmetcalf-liar.js         83663     1394.42
    promises-pacta.js                        OOM         OOM

    Platform info:
    Linux 3.11.0-19-generic x64
    Node.JS 0.11.12
    V8 3.22.24.19
    Intel(R) Core(TM) i7-3517U CPU @ 1.90GHz × 4
```