Fix parsing of action attributes

When parsing XML like this:

    <item>
      <action_key>123</action_key>
      <title>Foo</title>
    </item>

Nokogiri returns not only element nodes (item, action_key, title),
but also text nodes for the whitespace in between the tags.

The parsing code used to not to discriminate between the two types and
try to parse both the same way.
Fix passing of credentials from Action.fetch to ActionsFetcher

ActionsFetcher takes filters as the first parameter, and credentials as
the second. The old code passed credentials as filters.
Refactor ActionsFetcher into ObjectsFetcher

In order to support adding new item types.

Refs #3
Allow fetching supporters

Refs #3
Allow fetching supporter actions

Refs #3
Merge branch 'add_supporters_and_supporter_actions'

Conflicts:
	lib/salsa_labs/action.rb
	lib/salsa_labs/actions_fetcher.rb
	spec/salsa_labs/actions_fetcher_spec.rb