Use b.N in benchmarks

Currently, the output of `go test -bench=.` is not very useful
~~~
[martini] Started OPTIONS foo for 
[martini] Completed 200 OK in 154.09us
PASS
Benchmark_WithoutCORS	2000000000	         0.00 ns/op
Benchmark_WithCORS	2000000000	         0.00 ns/op
ok  	github.com/hariharan-uno/cors	0.014s
~~~
After this change, the output is informative
~~~
[martini] Started OPTIONS foo for 
[martini] Completed 200 OK in 150.767us
PASS
Benchmark_WithoutCORS	 1000000	      2221 ns/op
Benchmark_WithCORS	  500000	      6900 ns/op
ok  	github.com/hariharan-uno/cors	5.778s
~~~