Include usage analytics tracking

[zaus]

To improve my ability to understand what users are doing, and thus focus my efforts on future updates, I'd like to build as a "bundled plugin" a way to report generic usage analytics such as:

• plugin activation / deactivation
• number of services
• number of fields
• number of fields per service
• does it check for success?
• track that a submission occurred (but not the data)
• track how much information (number of fields) sent
• track that submission was success/failure

Allow 'opt-out' by disabling plugin.

Probably use Google Universal Analytics to take advantage of server-side posting.