Converted perl code to using promises, closes #420

[jhthorsen]

• Add "around_action" hook to catch rejected promises
• Add "reason" argument to Convos::Core->connect()
• Add Convos::Core::Connection->set_wanted_state_p() method
• Add Convos::Core::Backend->handle_event_p()
• Add Convos::Core::Backend->on()
• Add Convos::Plugin::Paste::File->handle_event_p()
• Add Convos::Plugin::Paste::File->to_message()
• Add a bunch of new tests
• Replaced all callback based methods with methods that return a promise
• Replaced Mojo::IOLoop::ForkCall with Mojo::IOLoop::Subprocess
• Replaced CONVOS_STEAL_NICK_INTERVAL with CONVOS_IRC_PERIDOC_INTERVAL
• Replaced "$self" in the POD with the name of the actual object, like
  $core, $user, ...
• Changes Convos::Core::Connection->wanted_state() to an attribute
• Changed default CONVOS_CONNECT_DELAY from 3 to 4
• Removed "get_user" WebSocket event
• Removed "participants" API endpoint
• Removed Convos::Core::Backend->emit_single()
• Removed next_tick() from Convos::Util
• Removed all blocking APIs
• Removed delay() helper
• Removed Mojo::IRC as a dependency
  • Mojo::IOLoop::Stream is now part of Convos::Core::Connection
  • IRC event handling is now in Convos::Core::Connection::Irc
• Removed Convos::Core::Connection->participants()
• Removed Convos::Core::Connection::Irc->participants()
• Moved logic from Convos::Core::Connection::Irc->connect()
  to Convos::Core::Connection->connect()
• Moved logic from Convos::Core::Connection::Irc->disconnect_p()
  to Convos::Core::Connection->disconnect_p()