Auto-disable more entities

@neovictor: There's a code of conduct for working within the Drupal community, and having an account on drupal.org in the first place indicates you (should have) read it and indicated agreement to its standards. Your third comment for the 2 years and 4 months you've had an account, is clearly in violation of this. Please clean up your attitude.

This makes a few small changes:

• Entities must be fieldable.
• Entities will not be considered as having view modes if they only have ones from the iCal, Diff or Token modules.
• Entities will be disabled by default, excluding the core Node, Taxonomy Term and User entities, to avoid them showing up in places they wouldn't be wanted.
• A new hook was added which allows further control over whether entities are supported: hook_metatag_entity_type_is_supported_alter()

Lets see what the testbots think of this. I've resolved all of the test errors locally and added a new update script to manually enable the node, taxonomy term and user entity & bundles.

AH! If I run the tests using PHP 5.3 the tests fail, but with PHP 5.6 (my local default) the pass. At least it's easy for me to change the local default :)

Ah! I worked out a flaw in my logic for hook_install(). It was skipping all entities that didn't have any bundles. However, when a site is first installed and node.module is installed there aren't any content types, ditto for taxonomy. So, what I had to deal with the three bundled entity types (node, taxonomy_term, user) separately and specifically enable them. I've also added hook_node_type_insert(), hook_node_type_delete(), hook_taxonomy_vocabulary_insert() and hook_taxonomy_vocabulary_delete() to update the variables accordingly.

Green! Woot!

Committed.