Add japicmp maven plugin

[dinomite]

Replaces PR 347.

Resolves issue #345

[dinomite]

@cowtowncoder Mainly adding you so you know this is actually going to be merged. We'll need to update the base version whenever methods are added or purposefully changed (I'll handling doing that).

[cowtowncoder]

Ok. This should help tracking changes, avoiding unintended compatibility affecting changes.

One note: make sure to merge this 2.12 -> 2.13, 2.13 -> master. I have merged a few PRs earlier but forgot to mention that there is no automatic process from merging from maintenance branches upwards toward master.
Merging itself is easy (git checkout 2.13; git merge 2.12; git push; ....) unless there are conflicts. I can help with this as necessary too but in general I think it best that whoever merges fix to a maintenance branch handles merging.

But one caveat here is that conflicts on pom.xml will result in omitting merge for it: this is due to setting in .gitattributes:

# Do not merge `pom.xml` from older version, as it will typically conflict
pom.xml merge=ours

added because otherwise there will always be bogus conflicts when releasing new versions from maintenance branches.

But downside is that changes to pom.xml need usually be manually merged: you can do a diff before committing to 2.12, or diff across versions.

[dinomite]

Got it, I'll keep up on merging 👍 (and do it for this branch now)