return byte slice directly from buffer

Updates #52
optimized buffer refill

No extra allocation when refilling the buffer
Closes #52
getSystemVar result validity doc
Grow buffer instead of using a tmp slice
grow buffer doc + fmt
grow buffer doc clarification
Merge pull request #55 from go-sql-driver/zero-copy

zero-copy buffer
don't fail on strict hosts
add dsn parameter to runTests
move charset tests down so all Test... are together
Merge branch 'master' into simple-benchmarks
use sane value for length of string selected; merged with current master