Initial work on extracting time support to st0603 module

[bradh]

Motivation and Context

This is a start on refactoring of common time stamp (ST0603) code into a separate module.
Relates to #97, but doesn't complete it.

Description

Pulls out the common code into a new module. This is pretty minimal at the moment, but does make more shared code.

Also added the "time status" byte from ST0603.5 section 7.4. We need this to drop into the ST0604 video timestamp (see #102). It also helped to get the test coverage up which made Coveralls happier after the migration to common code reduced the coverage numbers (same amount of tests, just less covered lines, so lower percentage?)

How Has This Been Tested?

Unit tests, with good coverage.
Ran the viewer application across a couple of test files - no problems noted.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.