Refactor + New Features + All Specs passing

This is a large set of changes but it is well tested, with spec count now at 286 (was ~150) and all specs passing. In a nutshell, I've greatly DRY'ed up the code and standardized methods, and added some useful features along the way.

Despite new functionality added there's about a 20% reduction in lines of code overall, and CodeClimate has also improved somewhat.

**New Features**

* Support matching timespan-type objects with distinct start and end times (previously only point-in-time matching was supported). By default, will match objects which either start OR end within the desired period; you can change this to AND using the `:strict => true`
* Add `:offset` option to all query methods, in order to offset the time the day begins/ends (for example supposing business cycle begins at 8:00 each day until 7:59:59 the next day)
* Add Time kernel extensions for fortnight and calendar_month

**Other Changes**

* Begin tracking CHANGELOG
* Drop official support for Ruby <= 1.9.2
* Add Ruby 2.1.0, Rubinius, and JRuby to Travis
* Decouple gem from ActiveRecord, and put ActiveRecord and Mongoid specific methods in ORM modules.
* Consolidate all normalization/parsing functions into new Normalization module
* Remove meta-programming methods, e.g. `send("by_week_#{time_klass}")`
* Make Chronic gem optional; use it only if user has included it externally
* `by_week` always returns a calendar week (i.e. beginning Monday or as specified by Rails setting), regardless of whether Date or Fixnum is given as a parameter.
* `by_week` and `by_calendar_month` now supports optional `:start_day` option (:monday, :tuesday, etc)
* Separate `by_calendar_month` into it's own class
* Rename `between` to `between_times` internally, as Mongoid already defines `between`. ActiveRecord has an alias of `between` to interface stays consistent.
* `by_weekend` can now take a fixnum (parsing logic is same as by_week)
* Two-digit year now considers 70 to be 1970, and 69 to be 2069 (was previously 40 -> 1940)
* Add myself as a gem co-author