Timezone bugfixes

[SijmenHuizenga]

Cross-timezone scheduling was broken. This is an attempt to fix that. To make sure this actually works in all cases, I've added a bunch of unit tests based on edge cases and reported issues. Furthermore I've split up the code a bit more to make it easier to understand and document. In this process I discovered more weird edge cases, which are now described as test cases.

Resolves #608 and #609 and #605 and #603

Everyone is invited to take a look and find more bugs / untested edge cases. Suggestions are welcome! I will leave this PR open until Saturday, after which I will merge & make a new release.

Todo:

• Add tests from examples in daylight saving bug #608
• Add tests for exotic timezones
• Add test that covers a full year of daylight saving time changes, both local timezone and remote timezone
• Add test for schedules that skips over daylight saving time changes, both local and remote
• Add tests for situations where both local and remote timezones experience daylight saving time changes between now and the next scheduled event
• Add separate tests for _move_to_next_weekday
• Add separate tests for _align_utc_offset

[coveralls]

coverage: 99.221% (-0.3%) from 99.474%
when pulling 252d451 on SijmenHuizenga:timezone-fixes
into 4386f45 on dbader:master.