tooltips for all skills appearing in a game

* Jenkins: http://jenkins.buttonweavers.com:8080/job/buttonmen-cgolubi1/247/
* Partially addresses #256

This may actually allow us to close #256, but i figured i'd check in with other interested parties before claiming it fixes it.  Reason: this fix does not create/populate help pages for skills, or allow skill descriptions to be pulled via API outside of the context of games.

What it does do is make a list of skills appearing in each game, and, for each skill, populates title text with the description of that skill.  On the backend, i structured descriptions of interactions between skills as an array, so, within a given game, each skill's tooltip prints only the set of interactions with other skills also in the game.  (Of course, *most* interskill interactions only apply to multiple skills appearing on one die, and as of yet this doesn't check whether skills appear on the same die, only whether they all appear within the same game.  But IMO that's good enough for a first cut, and it means we can be fairly generous in specifying interactions between skills, because only a subset of them will be printed in the context of any particular game.)

I believe i imported all of the text from https://github.com/buttonmen-dev/buttonmen/wiki/Tester-guide:-supported-skills except Option, which is not a die skill, with only trivial modifications.  That means that, if this is merged, the source code should become the authoritative source of die skill descriptions.
I see no examples of `+=` used for arrays anywhere in src/engine/, and the first thing i tried caused array key/array value issues.  I'd prefer not to change this, since i think it works as-is and avoids dealing with the vagaries of PHP around zero-length lists, etc.