Fix Timezone issues with UNTIL and DTSTART

[leaanthony-sc]

Fixes: #67

This PR contains the following fixes:

• Ensures that datetime format for DTSTART is the same as UNTIL if given
• If UNTIL is given and the datetime format is local, the time is interpreted in the timezone given in DTSTART if given (UTC is default)
• Ensures that if a timezone is specified in DTSTART, the UNTIL time is not in UTC

Added tests for each scenario addressed.