Long lines are split by character, not byte

[Sanqui]

Messages sent to IRC are split due to the IRC command length limit of 512 bytes. This appears to be done by the _splitLongLines function.

The current splitting method seems to count Unicode characters rather than bytes, resulting in (possibly large) parts of the message disappearing. It needs to be changed to count bytes of the string encoded in utf-8.

Noticed with discord-irc, issue reactiflux/discord-irc#199 (contains a test log). discord-irc is using node-irc version 0.5.2.