Add pt-table-usage

Add the mk-table-usage tool, with some documentation clarifications. This is for Percona customer issue 22175.

One thing we should clarify in the documentation is that it is possible to store the usage information in a table, or output it in a different format such as JSON, but that is not implemented yet and can be done upon request. Anything else?

Daniel's note: I've left that ^ for another release, or for a blog post or something else. For the time being, I just want to get pt-table-usage in 2.1 and out the door.