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.
Merge pull request #4 from psyho/fix_credentials_passing_in_action_fetch

Fix passing of credentials from Action.fetch to ActionsFetcher
Merge pull request #5 from psyho/fix_action_parsing

Fix parsing of action attributes
Refactor ActionsFetcher into ObjectsFetcher

In order to support adding new item types.
Allow fetching supporters
Allow fetching supporter actions
ObjectsFetcher.fetch must call fetch
Documentation cleanup.
Bump to 0.0.3.

Supporters and Supporter Actions! Thanks to @psyho for some great new
capabilities.
Add Ruby 2.1.1 to Travis build matrix.