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Access a specific cell of the board (see #3)

So you can do things like:

cell = board['my column']['my lane']
cell = board.column('my column').lane('my lane')
Get cell tasks (see #3)

* Cell#tasks to access a cell's tasks
* Task#column & Task#lane so tasks can be filtered easily
* a Task needs a column and a lane to be initialized
* it also needs the parent board to be passed to #initialize
* the board is also passed to Column#initialize
Rake test task wasn't picking up all the specs
Cassette for the column specs that were failing before
Get column tasks (see #3)
Caching column tasks (see #3)

And providing Column#tasks! to get fresh ones
from the API if needed.
Get lane tasks (see #3)
Caching lane tasks (closes #3)