Integrated StorageFormatManager into Leaf.__repr__.
Updated keyword in IOManager.count_function_calls().

`sortby` needs to be "cumulative" rather than "cum".
Integrated distutils.version.LooseVersion into version checking.

LooseVersion allows for greater-than/less-than comparisons on version number
strings:

    >>> from distutils.version import LooseVersion
    >>> LooseVersion('2.7.2') < LooseVersion('2.7.6')
    True
    >>> LooseVersion('2.7.6') < LooseVersion('3')
    True