Cleanup _gatherMessageValues and _assignMessageVariablesToTemplate in Contribution

I'm working my way to create custom message templates to export contributions.
The values passed to the template are defined by the _gatherMessageValues and _assignMessageVariablesToTemplate functions in CRM_Contribute_BAO_Contribution

I've noticed we're missing some needed elements in the templates, and i'm adding those needed. See:
https://issues.civicrm.org/jira/browse/CRM-17125
https://issues.civicrm.org/jira/browse/CRM-17106
https://issues.civicrm.org/jira/browse/CRM-17134

This starts to feel hacky in a way. It seems better to clean this up completely, and add more values in a structured way. However we probably should discuss what would be a good format of the values array, and how these should be passed to the template.

As a start i would suggest something like this.

$values = array(
  // the different contribution values
 'softcontributions' => array of softcontributions,
  'event' => if event, add event object
  'event.participants' => participants of event
  'memberships' => memberships
);

Probably related could be to clean up the customfield format of an object, to return values in the same format as normal fields.