Added payload_contains program
Added payload_contains program

Implementation is copied from route_contains mutatis mutandis.

Use memmem to compare routes and payloads.

memmem needs `_GNU_SOURCE` to be set. cf:
http://man7.org/linux/man-pages/man3/memmem.3.html

circonus-labs/fq,circonus-labs/fq,circonus-labs/fq,circonus-labs/fq,circonus-labs/fq,circonus-labs/fq