Add a Matches() method to support attribute matching.

Attribute matching is necessary in order to support DICOM queries.
The rules for attribute matching are given in DICOM part 4.  This
patch is more-or-less complete, except that it does not yet do
timezone adjustments for date and time comparisons, nor does it do
any normalization for name comparisons.
Add length check to Compare to simplify calling code.
Add CharacterSet check to "char" specialization of Compare.
Add a special case for comparing values with VR of AT.

If an AT attribute is part of a query, I can only guess that it
is there by mistake.  The safe approach is for it to always match.
The check in CompareEach was reversed.
When comparing dates, make sure to ignore space padding.
Fix some issues with sequence matching for queries.
Fix conversion of matching patterns to UTF8.
Add tests for value matching for queries.

Here are the tests that uncovered the bugs that were fixed in the
last few commits.
When normalizing date time, only copy digits and decimal point.
Convert DICOM datetime to UNIX time and back again.
Add test for vtkDICOMUtilities.