Fix bugs: index merge, image tag, g prefix, ignored tags

[consideRatio]

Key issue fixed: index merge

Closes #62 that followed an issue introduced by #52 by implementing #62 (comment) and catching some past mistakes of mine along the way. Fixing #62 is urgent as chartpress 0.4.2 is breaking expected functionality of chartpress.

Additional issues fixed

• image tag: Found and fixed a bug from Pre-release replaced by later builds #62 relating to the use of - instead of . as separator for image tags, which was not according to plan or inline documentation.
• g prefix: Stripped away a "g" prefix that follows from use of git describe.
• Ignored tags: Fixed lacking ability to handle tags after latest modification commit, and renamed function to reflect this.

Other

• Deleted a remnant function of past logic: last_modified_date(*paths).
• Added pytest tests for simple to test functions _get_identifier and _strip_identifiers_build_suffix

[consideRatio]

Notes to self

• refactor the now chart/image common logic into a function.
• create a function to identify build section and strip it, and stop sending two return values in a tuple as i started doing in this PR

[minrk]

Excellent!

[consideRatio]

@minrk I had documented in the travis script that rebuilds shouldn't be triggered on running chartpress on z2jh, but it is currently doing that. This is because of the changed image tags that are not yet pushed (7927c93).

[minrk]

Not sure if it was this PR or not, but I'm getting the same effective last-changed commit on all images, regardless of when the image's files were last changed.

[consideRatio]

f41990c - this is probably the relevant change commit.