Adds a notification plugin, and a few tweaks to help support this

[jghibiki]

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Adds a notification plugin, with back-ends for pushbullet, email, and slack (the underlying library notifiers supports additional platforms).

Also updates the Server API system to add a ServerResponse object to better standardize the responses from the plugin/builtin server api hooks.

Also refactors the IntenEvent system to have it's own builtin. Added a handler that will take an Intent object and convert it to a typed Event subclass object that can be submitted to the event bus. This allows plugins and builtins to be loosly coupled as a builtin or plugin can create an intent which will be converted to a required typed event that another builtin or plugin can handle.

e.g.

IntentEvent({"type": "msa.plugins.my_plugin.events.MyCustomEvent", "context": { <my event init data}) 
  -> IntentHandler 
    -> MyCustomEvent instance 
      -> event bus

For example to trigger a notification via the notification plugin via the cli (requires that the notification plugin is configured, and configured to be loaded in the msa_config.json):

# Tightly coupled version, requires being able to import the SendNotificationEvent
from msa.plugins.notifications.events import SendNotificationEvent  
from msa.plugins.notifications.notification_providers import NotificationProvider
a = SendNotificationEvent()
    .init({
        "provider": NotificationProvider.pushbullet.value, 
        "title": "test!", 
        "message": "this is a sample message"
    })
    .network_propagate()

# Loosely coupled version:
from msa.builtins.intents.events.IntentEvent
a = IntentEvent()
    .init({
        "type": "msa.plugins.notifications.events.SendNotificationEvent",
        "context": {
            "provider": "pushbullet",
            "title": "test!",
            "message": "this is a sample message"
    }}
   .network_propagate()

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[jghibiki]

Test Coverage is up to 70%, new plugins are fully tested.

[jghibiki]

@dekarrin Any idea why the build would be failing as it is on the git clone step?

[jghibiki]

Hm, well the clone step worked this time. Must have been a temp. problem.

[jghibiki]

:tada: build finally succeeded!

[jghibiki]

@dekarrin whenever you have some free time, i think this is ready for review.