Add rake task for running plugin tests.

This makes it possible to run Discourse tests with plugins loaded by setting `LOAD_PLUGINS=true`, as well as to run specs defined in a `spec/` directory inside plugins by running either `rake plugin:spec` for all plugins or `rake plugin:spec[pluginname]` for a specific plugin.

Since most plugins aren't going to be complicated enough to require sub-directories inside of the `spec/` folder I don't recursively glob, all specs need to be directly in the `spec/` directory.

So it becomes easy to write tests for plugin functionality like this one:

```ruby
require 'spec_helper'
require 'post_creator'

describe PostCreator do
  let(:user) { Fabricate(:user) }

  context "new topic" do
    it "poll cannot be created without a list of options" do
      post = PostCreator.create(user, {title: "Poll: This is a poll", raw: "body does not contain a list"})
      post.errors[:raw].should be_present
    end
  end
end
```