Add documentation for protocol methods
Use constants for NewResponse
Improve WriteBulk performance

The performance improvement is astonishing: in the previous
commit (87331b7928d2074ce0b4baa7e3397f5110d605b2) benchmarking WriteBulk
reports the following:

    > go test github.com/inkel/gedis -bench=WriteBulk
    PASS
    Benchmark_WriteBulk      5000000               423 ns/op
    ok      github.com/inkel/gedis  2.583s

After applying this changes, performance commit almost doubled, as you
can see in the following results:

    > go test github.com/inkel/gedis -bench=WriteBulk
    PASS
    Benchmark_WriteBulk     10000000               243 ns/op
    ok      github.com/inkel/gedis  2.781s

Perhaps this might not be the optimal method to do it, but it's a good
way to learn about Go internals like `make` and `append`, and to improve
the speed of the library at the same time.