Rename timezone.jl to Timezone.jl
Added parameterized abstract type Offest{n} to represent generic offsets. This is a pretty rough 1st draft; need to think about what units to store the type as, convenience functions for offsetting (offset()?), and how to better print the offset.
Updated tzdata files
Tweaked perf and made isleap() to work with a Year type.
Rebase
Reworked the Date and DateTime type safety conversions/promotions. This avoids segfaults for inter-Calendar or inter-TimeZone operations.
Fixed Date show/string function to display negative dates correctly (i.e. convert(Date,-1) is -0001-12-31 instead of a non-existent year 0).
For DateTime differences (-), return # of microseconds as Int64 for now. If we get the SubSecond{p} type at some point, we can change this.