New game mode: Budget decks

From reddit:
https://www.reddit.com/r/XMage/comments/4hm01s/budget_game_mode/

"A game mode where a deck had to have a total 'cost' under a set value. I know half the fun of this is to play decks that you could never run in paper magic, but trying to brew off beat decks and just getting wrecked by same (nearly) $1,000 superfriends deck over and over is getting old."

Looks like it would need a few things to be done:
1) A source to determine current card prices - and calls to it regularly, or cached on a daily basis to get the latest information for all the cards. Then when someone tries to create/join the game the deck is checked against this information to see if it exceeds the Max.
2) When creating the game, a Max Amount field so they can limit it to decks that are < $100 or < $1000 etc

No files, tests, or implementation entry points are named. Start by locating the game-creation and deck-validation entry points, then investigate a card-price source and the proposed caching schedule. Done means a configurable maximum is enforced when creating or joining games using current card prices.

Written by the indexing model from the issue text.