Handle daylight savings time in WeekRange#from_time

Fixes issues with daylight savings time. (issue #60)

===

**Before PR**:

```ruby
Time.use_zone('America/Los_Angeles') do
  range = DoubleEntry::Reporting::WeekRange.from_time Time.zone.parse('Mon, 10 Mar 2014')
  range.start
end

# => 2014-03-03 00:00:00 -0800
```

**After PR**:

```ruby
Time.use_zone('America/Los_Angeles') do
  range = DoubleEntry::Reporting::WeekRange.from_time Time.zone.parse('Mon, 10 Mar 2014')
  range.start
end

# => 2014-03-10 00:00:00 -0700
```