usage statistics

[ilveroluca]

It's important that we track Life Monitor usage. I propose we implement statistics collection in LM to measure things such as:
* number of user accounts
* number of registered worfklows
* workflow suites
* workflow test instances
* number of visits
* number of any actions taken by LM (e.g., executed repository checks)

Also, I propose we save the counters each month, and implement an API call to retrieve the statistics. If we can implement this sort of functionality with off-the-shelf tools (e.g., google analytics), all the better.