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:

              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)
Merge pull request #235 from vmg/benchmark

Add a basic benchmark file