Makes BlockBlackFlower place-able on custom soils

Changes canBlockStay so that it checks if the block below it can sustain the plant, instead of checking if the block is dirt or grass. This means that the flower can be placed on custom dirt and grass blocks such as the ones in Enhanced Biomes.
Merge pull request #12 from SMEZ1234/patch-1

Makes BlockBlackFlower place-able on custom soils