add more benchmarks

move benchmarks back into own file
add benchmark by bradfitz
add benchmark suite from go-sql-driver

This should ease testing of speed and allocations, we can stay inside the repository and don't have to worry about library versioning.

I used this to check the mem-pool PR.
Results for `go test -bench=. -benchmem -benchtime 10s` (OS X 64, Go 1.1):

**this version as is**
```
PASS
BenchmarkQuery	 1000000	     25922 ns/op	     585 B/op	      24 allocs/op
BenchmarkRoundtripTxt	  100000	   1038977 ns/op	  207769 B/op	      14 allocs/op
BenchmarkRoundtripBin	  200000	    855491 ns/op	  510250 B/op	      26 allocs/op
BenchmarkSimpleQuery	  100000	    209500 ns/op	    8801 B/op	     412 allocs/op
BenchmarkPreparedQuery	  100000	    227128 ns/op	    5639 B/op	     318 allocs/op
BenchmarkAutoQueryRow	  100000	    169876 ns/op	     733 B/op	      26 allocs/op
BenchmarkSimpleQueryRow	  200000	    109934 ns/op	     553 B/op	      17 allocs/op
BenchmarkPreparedQueryRow	  200000	    102223 ns/op	     634 B/op	      25 allocs/op
BenchmarkSimpleExec	  500000	     46630 ns/op	      80 B/op	       3 allocs/op
BenchmarkPreparedExec	  500000	     43215 ns/op	     150 B/op	       7 allocs/op
```

**this version merged with mem-pool**
```
PASS
BenchmarkQuery	 1000000	     27478 ns/op	     425 B/op	      19 allocs/op
BenchmarkRoundtripTxt	  100000	   1033229 ns/op	  189967 B/op	      12 allocs/op
BenchmarkRoundtripBin	  200000	    847945 ns/op	  408143 B/op	      22 allocs/op
BenchmarkSimpleQuery	  100000	    210917 ns/op	    8660 B/op	     410 allocs/op
BenchmarkPreparedQuery	  100000	    220103 ns/op	    5530 B/op	     316 allocs/op
BenchmarkAutoQueryRow	  100000	    169921 ns/op	     524 B/op	      20 allocs/op
BenchmarkSimpleQueryRow	  200000	    111585 ns/op	     411 B/op	      15 allocs/op
BenchmarkPreparedQueryRow	  200000	    101977 ns/op	     458 B/op	      20 allocs/op
BenchmarkSimpleExec	  500000	     48310 ns/op	      48 B/op	       2 allocs/op
BenchmarkPreparedExec	  500000	     42203 ns/op	     134 B/op	       6 allocs/op
```