Adds digest consumer funtionality

Fixes #4 

- Create include file (campaign-markup.inc) from Mailchimp HTML template to format campaign entires.
- Add related merge_var values to campaign-markup.inc
- Increase batch / page size to 500
- send global_merge_var values to merge_var function as Mandrill currently does not support replacing global merge var values in individual user merge var submissions.
- Limit the number of campaigns in message to five.
- Move `$ch = curl_init()` outside of request loop.
- Skip user campaigns that don't have an entry in the campaign api.
- Add comments for $to[] assembly functionality.
- add `width="264" style="max-width:800px;" class="mcnImage"` to campaign image to allow mobile scaling.
- Add functionality to replace merge vars with campaign content:
    - *|CAMPAIGN_IMAGE_MARKUP|*
    - *|CAMPAIGN_TITLE|*
    - *|CAMPAIGN_CALL_TO_ACTION|*
    - *|CAMPAIGN_LINK|*
    - *|DO_IT_TITLE|*
    - *|DO_IT_BODY|*
    - *|FACT_PROBLEM|*
    - *|FACT_SOLUTION|*
- Create campaign divider markup (HTML) include file: `campaign-divider-markup.inc`
Could this 5 be set as maybe a const value in this class? Or at least comment this line. At a glance I don't know what this 5 is meant for.