The module does not build with any version of python<=2.6 and the tests will not run in any version of python<=2.7

Currently the `datasift-python` module does not build with Python 2.5, and the tests do not run with Python 2.5, or Python 2.6. This set of patches fixes various syntax and module incompatibilities between the different versions of Python.
##### Module changes:
 - The `simplejson` module is used as a fall-back if `json` cannot be imported. `simplejson` is included in `python>=2.6` as the standard library `json` (http://docs.python.org/2/whatsnew/2.6.html#the-json-module-javascript-object-notation).
 - The `unittest2` module is used instead of the standard library module `unittest`. `unittest2` provides the same functionality as the Python 2.7 module `unittest`. 

##### Syntax changes:
 - Using `as` is not possible in `python<=2.5`, all uses of `as` have been changed to the legacy `,` syntax.