Add median and percentile graphite methods

[yehzhang]

No description provided.

[coveralls]

Coverage decreased (-11.6%) to 50.183% when pulling 31bc0aa on yehzhang:simon/develop into 20a9fb1 on klen:develop.

[coveralls]

Coverage decreased (-1.0%) to 60.806% when pulling 277fb29 on yehzhang:simon/develop into 20a9fb1 on klen:develop.

[coveralls]

Coverage increased (+1.5%) to 63.325% when pulling f5f4d32 on yehzhang:simon/develop into 20a9fb1 on klen:develop.

[garrettheel]

Thanks for the changes, looks great! Did you consider using p<rank> as the syntax instead? E.g p50.