[irc] Only split args with spaces, fixes #406

Before this commit, that `argstr.split()` was using Unicode whitespace in channel names as a delimiter, so a message to the channel `#hi 123` was being interpreted as:

````
[u'PRIVMSG', u'#hi', u'123', u'hello']
````

This commit fixes the problem by only allowing spaces, and now Willie interprets the channel name correctly:

````
[u'PRIVMSG', u'#hi\u2005123', u'hello']
````