Adding supports for time zone (TZ).

See Issue #78.

This is a mirror commit of Yuriy Silvestrov's commit 808f3fa472fbb1cedab8d10cb6efa5265721a0bc (See reference 1).
Attribution have been correctly given. The reason for a separate commit is because Yuriy Silvestrov change modified the whitespace (See reference 2).

Variations:

- Variable zone is renamed to timezone.
- Tries to follows project convention closely.
- Generated min version.
- Did not brought over renaming of css class 'switch'.

References
1. https://github.com/ysilvestrov/bootstrap-datetimepicker/commit/808f3fa472fbb1cedab8d10cb6efa5265721a0bc?w=1
2. https://github.com/tarruda/bootstrap-datetimepicker/issues/78#issuecomment-15020901