Weekday fixes

In continuation of #37 and #48.

**IMPORTANT note:** the changes itself in MTDates seems to be reasonable to me, but when I brought this version of MTDates to the production app I'm working on I see lots of failing unit tests around date calculations :( I'm going to investigate further on another day when I have more time. To be honest I'm not sure I will be able to do this until next week.

If anyone has time to test this pull request and/or do code review - I'll very appreciate your support.

```ruby
pod 'MTDates', :git => 'https://github.com/yas375/MTDates.git', :commit => '9a56b67'
```

@Alex9779 could you please take a look at the fixes I done on top of yours? Thanks in advance!
Actually such tests doesn't seem to be right to me... If `mt_startOfCurrentWeek` returns a date from wrong week (i.e. from the next week), then the test still passes because it just compared just names of the days. Maybe we should remove them and to leave only updated tests...