Preserve timezone when calling #fix_property_dates.

Before this patch, all dates sent to Mixpanel are considered on
UTC+0. This patch adds the timezone to the string format used to
convert DateTime objects to strings before sending them to Mixpanel.
I have confirmed that Mixpanel is able to parse this format.
See #49.
Write dates as UTC rather than with offsets
Version bump
Add optional_params to people#delete_user to delete users and ignore alias
Added regression test and ignore_alias test
Merge pull request #53 from eric-surfeasy/delete_aliased_user

Add optional_params to people#delete_user to delete users and ignore alias