Added Offset support to Local

[araspik]

This would be really useful when trying to convert DateTime<Local> to DateTime<FixedOffset>, allowing this simple code:

local_datetime.with_timezone(Local.fix())

instead of the much longer current solutions.
Note that no tests are added; This was a quick solution (even though I dug through both time and chrono's docs for a bit).
Improvements are definitely possible, but this is a good start.

[araspik]

It seems that Travis CI tests on Mac (w/ Rust 1.13.0) are broken (something about unions)?

[mqudsi]

@araspik #271 fixes the root issue by adding Into and From for all the datetime combinations, that should probably suffice for your needs?

[ErichDonGubler]

Wait, #271 actually just sets the seconds offset of the FixedOffset to be 0, doesn't it? Is that actually what we want here?

[quodlibetor]

Yeah #271 allows converting Local to e.g. Utc, but this would allow converting to a FixedOffset that is the same as the Local Offset. If this got some tests I'd be happy to merge it.

Rebasing on top of master will fix the 1.13 tests.