Fix date parsing (missing timezone)

When you parse any date (ISO8601 or RFC1123), the `boto.utils.parse_ts()` will always give you instance of `datetime.datetime` **without** `tzinfo` so you can only predict it's UTC, which is true in most cases, but it complicates any further conversions between timezones.

In simple terms the function you can't reconstruct the same date passed into this function as it relies on `datetime.strptime()` which ignores parsed timezones in the string too:

https://docs.python.org/2/library/datetime.html#strftime-strptime-behavior

`datetime.strptime(date_string, format)`
is equivalent to
`datetime(*(time.strptime(date_string, format)[0:6]))`

This patch is fixing it.
See attached test suite.

This solves #875