LWRP Refactor

The `README` changes should tell you all you need to know, so this is going to focus on what you don't need to know, but might be useful.

  - Using a custom partial API (v1, v2) found in `libraries/`  
    Ideally this would be replaced with a dedicated API gem, but I couldn't find one that met the needs of this cookbook.
  - Supports non-LWRP based usage, but does introduce breaking changes; suggest version bump to `2.0.0`
  - The alert LWRP sort of cheats and leaves it to the user to sort out v1/v2 differences in alert creation, ideally this would be abstracted away, but time/effort etc
  - `agent_key` is cached in the node's data, override it to `nil` to force a new read from the API
  - intended usage is to remove all alerts, and then reapply them because tracking alerts is impractical, this provides two benefits;
      1. old alerts get removed
      2. alerts do not get duplicated
  - `:update` and `:clear` are separate actions, so that it's possible to configure the agent without wiping existing alerts

Here's an example of v1 usage:
```rb
include_recipe 'serverdensity'

# unless `device` is passed here (see README), the name is used to lookup the node by name,
# for devices created this way, it's more reliable than hostname
serverdensity node.name do
  account 'idio.serverdensity.com'
  username "chef"
  password ""
  # dual action, update the device, and clear all alerts (without clear, alerts rapidly start to
  # duplicate as there's no easy way to track them)
  action [:update, :clear]
end

serverdensity_alert 'high-load' do
  # metadata gets sent straight to the API, errors are logged as Chef warnings so as not to
  # interrupt an otherwise successful run
  metadata(
    :userId => ['group'],
    :notificationType => ['email'],
    :checkType => 'loadAvrg',
    :comparison => :>,
    :triggerThreshold => 3
  )
end
```

and v2:
```rb
include_recipe 'serverdensity'

serverdensity node.name do
  account 'idio.serverdensity.com'
  token "000000000000000000000000000000"
  action [:update, :clear]
end

...
```

Happy to answer any questions you may have, cheers!