Improve efficiency of readBulk buffer reading

[jpfuentes2]

The previous implementation was reading 1 byte at a time and then
copying it to a target buffer. Now it reads as many bytes as
possible into the target buffer up to the byte length.

Also changed num_bytes to numBytes as it was probably a Ruby typo

[inkel]

It's much faster indeed!

~/dev/go/src/github.com/inkel/gedis (master:v0.0.5) >> go test -bench=readBulk .
PASS
Benchmark_readBulk       1000000              1109 ns/op
ok      github.com/inkel/gedis  2.518s

~/dev/go/src/github.com/inkel/gedis (master) >> git co jpfuentes2/improve_readBulk_buffering

~/dev/go/src/github.com/inkel/gedis ((21d5f6f...)) >> go test -bench=readBulk .
PASS
Benchmark_readBulk       5000000               713 ns/op
ok      github.com/inkel/gedis  11.932s

That's a 26% improvement!

Now I recall the reason why I did it like this was because I wanted to check for EOF, but Read already does that, so awesome work!

Funny thing: I did implement it like this in the server/reader.go version of readBulk.

Thanks @jpfuentes2!