Fix utf8 lengths
Length should always be the number of bytes in the string,
not always String.length.
If a string is passed to nsPayload, convert it to a Buffer
so we can compute the number of bytes in the string.
Add npm package
Fix writing empty netstring
Test empty payload
Test empty payload is incomplete
Test payload length with leading colon
Support serialization to Buffer objects.
Support Buffer payloads in nsWrite().