Informative error message around DataShape(str)

In [1]: from datashape import DataShape, dshape

In [2]: DataShape("5 * int")
TypeError: DataShape constructor for internal use.
Use dshape function to convert strings into datashapes.
Try:
    dshape('5 * int')

In [3]: dshape('5 * int')
Out[3]: dshape("5 * int32")

cc @chrish42

Refer https://github.com/ContinuumIO/blaze/issues/807