Replace json library with yajl

Yet Another Json Library is a replacement for standard JSON library,
which parses json faster and with less memory usage.

Tests run on my machine:

with json library:

> Finished tests in 0.077247s, 776.7292 tests/s, 789.6747 assertions/s.

with yajl library:

> Finished tests in 0.049741s, 1206.2484 tests/s, 1226.3525 assertions/s.