Respect mock instance in createFromFormat with partial format

[imbrish]

Let's say current time is:

Carbon::now(); // 2018-05-10 16:22:30 America/Toronto

We set the mock instance to:

Carbon::setTestNow('2013-09-01 05:10:15 America/Vancouver');

One would expect that when using Carbon::createFromFormat with a partial format, the missing part will be filled from the mock:

Carbon::createFromFormat('Y-m-d', '2016-06-10'); // 2016-06-10 05:10:15 America/Vancouver
Carbon::createFromFormat('H:i:s', '10:20:30'); // 2013-09-01 10:20:30 America/Vancouver

However as of now it is filled from the actual time instead:

Carbon::createFromFormat('Y-m-d', '2016-06-10'); // 2016-06-10 16:22:30 America/Toronto
Carbon::createFromFormat('H:i:s', '10:20:30'); // 2018-05-10 10:20:30 America/Toronto

This PR fixes that issue, by prepending mocked time to the input. It works with many fancy formats, including unix epoch reset flags !, |. See new tests for more details.

Changes are not backwards compatible, however that's often the case when fixing bugs, which I would assume the old behavior to be. Worst case scenario would be that we break some odd monkey-fixed tests that circumvented the problem by comparing against the real time. We don't need to worry about someone using setTestNow in the business code, right?

The PR includes two additional changes to preserve current behaviors:

1. Last errors applied to resulting instance come from parsing original (unmodified) format.
2. Microseconds in the Carbon::create method are explicitly set to 0 by using format !Y-n-j G:i:s instead of previous Y-n-j G:i:s.

Bonus:

1. Fix creating timezone from an offset for PHP below 5.5.10.

Fixes #1280.