macaroon: add some benchmarks

For the record, here are the current results:

BenchmarkNew	  500000	      4283 ns/op
BenchmarkAddCaveat	  500000	      3402 ns/op
BenchmarkVerifyLarge	   50000	     67773 ns/op
BenchmarkVerifySmall	  500000	      6219 ns/op
BenchmarkMarshalJSON	  500000	      3644 ns/op
BenchmarkUnmarshalJSON	  200000	      8512 ns/op
Merge pull request #8 from rogpeppe/007-benchmark

macaroon: add some benchmarks