Added support for multiline headers.

[ladenedge]

Bit of a tradeoff here. I decided to just send additional lines with no separator from previous lines. One could send newlines with all header values, the way part data works, but that could possibly be a breaking change. One could also add additional states, rewind the index and add newlines only on multiline headers, but that seemed like too much complexity.

This patch is meant to resolve #14.

[ghost]

Hi! How this commit should looks like at the moment?