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.

To test import some fake images:
```
bin/omero import FAKE_NO_DATE.fake
bin/omero import FAKE_WITH_DATE\&acquisitionDate\=2014-07-08_12-00-00.fake
bin/omero import FAKE_WITH_BROKEN_DATE\&acquisitionDate\=2014-NO-Se-ne-.fake

bin/omero hql "from Image"
 # | Class  | Id | details         | name                                                      | fileset    | acquisitionDate          
---+--------+----+-----------------+-----------------------------------------------------------+------------+--------------------------
 0 | ImageI | 1  | owner=2;group=3 | FAKE_NO_DATE.fake                                         | FilesetI:1 |                          
 1 | ImageI | 2  | owner=2;group=3 | FAKE_WITH_DATE&acquisitionDate=2014-07-08_12-00-00.fake   | FilesetI:2 | Tue Jul  8 13:00:00 2014 
 2 | ImageI | 3  | owner=2;group=3 | FAKE_WITH_BROKEN_DATE&acquisitionDate=2014-NO-Se-ne-.fake | FilesetI:3 | None                     
(3 rows)
```

/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_/