Parse empty struct

    In [1]: import datashape

    In [2]: datashape.dshape('{}')
    Out[2]: dshape("{  }")

    In [3]: datashape.dshape('5 * {}')
    Out[3]: dshape("5 * {  }")
Merge pull request #114 from mrocklin/accept-empty-struct

Parse empty struct