Add a basic benchmark file

Fix the benchmark rake task which was useless adding a super simple
benchmarking file measuring performances with other markdown parsers:

  * Kramdown
  * BlueCloth
  * RDiscount

Results example of the benchmark:

|      lib       |       user       |    system  |    total         |         real         |
-----------------|--------------------|-----------------|-------------------|-------------------------
| Redcarpet |  1.780000     |   0.010000 | 1.790000     |  (1.802302)      |
| BlueCoth   | 14.150000    |   0.050000 | 14.200000   |  (14.515556)    |
| RDiscount |  6.970000     |   0.060000 |  7.030000    |   (7.147544)     |
| Kramdown |  320.840000 |   1.610000 |  322.450000 |   (325.181537) |

I think this doesn't need a changelog entry but let me know if you think so.