Big refactoring of request statistics code

Out of RequestStats, factored out a StatsEntry class which represents a single stats entry (URL). Previously the RequestStats class was actually doing two different things: On the global class level, it was holding track of the aggregated stats from all requests, and on the instance level holding the stats for single stats entries.

Now RequestStats should be instantiated and holds the global stats, as well as a dict of StatsEntry instances which holds the stats for single stats entries (URLs)
Removed old code that is no longer used
Added tests for the web UI
Updated test for wrong_url because for some strange reason I get a MissingSchema exception (instead of InvalidSchema) when using telnet:127.0.0.1, even though it apparently is working on TravisCI. But telnet://127.0.0.1 should be invalid as well so this change should be OK.
Merge pull request #74 from locustio/stats-refactor

Refactoring of statistics