GADDAG support

[ipkiss42]

The current dictionary format in Eliot is the DAWG, described here: http://www.wutka.com/dawg.html.
The standard Appel & Jacobson algorithm is based on this data structure.

A faster implementation could be implemented, using a GADDAG dictionary format, as described by Gordon here: http://nongnu.org/eliot/download/aj.pdf

The first step of the support would be to handle GADDAG format in the CompDic and ListDic classes. The second step would be to integrate it with the search algorithms (dictionary search, and search on the board).

Eliot should still be able to support the DAWG format (it can distinguish the format with a field in the Header class).