Make crypto accept and emit buffers

[thiagoarrais]

Pull request rebased to current master. Only minor modifications were needed.

Fixes #1393

[jcayzac]

I find 'binary' and null quite redundant and confusing. Shouldn't it just return a buffer if encoding is set to 'binary' ?

[thiagoarrais]

I agree (I would even say that binary encoding should be ditched altogether in favor of buffers), but there is an issue with backwards compatibility.

[langpavel]

@thiagoarrais
You can now use pseudo 'buffer' encoding in crypto

@bnoordhuis Close?

[indutny]

I'll close it, it's irrelevant now.