Fix unicode problems in JSON encoding, fixes maknz#4.

Not sure why json_encode() outright broke with a string like 日本語, it should have just returned escaped unicode like \u<something>. But telling json_encode() to not escape unicode and leave as-is seems to fix the problem so ¯\_(ツ)_/¯

Set correct value for channel in constructor

Fixes maknz#3. This was a typo where username from the config was being used for the channel provided through the constructor. This affected users wanting to use the default channel in the config rather than using an explicit to() at runtime.

Merge pull request maknz#2 from maknz/develop

Rework the send method and implement attachments and icons

Update README with requirements

Usage instructions