Add Minefield class

has init_board() and init_mines() methods
Add scss/html skeletons
Add click/rightclick handler
Add basic routine skeletons
Merge branch 'feature/base' into develop

Basic skeletons
scss: Add td classes and images
Generate @nears table

@nears[x][y] contains # mines near (x,y).
Implement press method
Factor out near_positions method
Implement expand method
Remove redundant blobs in init_board
Rename @nears to @near_mines
Merge branch 'feature/expand' into develop
Do not flag if game is over
Implement flag method
Expand if flagged well
Expand even if there are too many flags

Can be occured if flags are incorrectly set.
This may lead mine explosion - maybe two or many.
Stop expanding if mine has exploded

Fixes simultaneous explosion.
Merge branch 'feature/flag' into develop
Factor out @{get,set}_class methods