Add benchmark tests. As used by other database/sql drivers.

Useful for testing performance.

My results:
```text

go version devel +d448734315d3 Mon Mar 25 08:20:22 2013 +0100 linux/amd64
BenchmarkExec           100000      63531 req/s
BenchmarkQuery           50000      28824 req/s
BenchmarkParams          50000      27641 req/s
BenchmarkStmt           100000      50275 req/s
BenchmarkRows             2000        913 req/s
BenchmarkStmtRows         2000        943 req/s
PASS
ok  	github.com/timob/go-sqlite3	18.374s
```