Add acquisition date to fake reader.

Due to changes in https://github.com/openmicroscopy/openmicroscopy/pull/2847 making the image acquisition date optional one test using a fake file is now failing, see [1]. The best solution here would be to allow fake files to optionally have an acquisition date. This PR adds that facility and would seem to provide more generally useful functionality.

I have used the `FILENAME_FORMAT` as this offers the simplest way of adding a date to the filename but it doesn't deal with either milliseconds or timezone. I don't think that is too much of hindrance given the scope of this PR - and not wanting to touch DateTools!

I have not added a unit test as not all values are tested but as the code deals with a couple of conditions but I am happy to add tests if the PR is otherwise acceptable.

/cc @mtbc @sbesson @joshmoore

1. http://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-integration-python/201/testReport/test.integration.clitest.test_search/TestSearch/test_search_dates_data2_/