Resource fixes

1.  Can pass dshape and schema kwargs from Table to resource as in 

        Table('foo.csv', schema='...')

2.  resource('/path/to/*.csv') now enforces that the schema of the first item should be the same on all of the items.
`foobar_a.csv` has schema `'{_1: float, _2: int}'`.  This is just enforced across everything that comes afterwards.

So, for example, when we parse the second csv file we'll end up getting to a point like 

    nd.array(['3', '3'], dtype="{_1: float, _2: int}")