Avoid unicode LookupError

`six.text_type` is `unicode` on Python 2 and `str` on Python 3. Both of these
functions accept multiple parameters: the second is the `encoding` parameter.
According to the documentation: "The encoding parameter is a string giving the
name of an encoding; if the encoding is not known, `LookupError` is raised."
Because `LookupError` does not inherit from `TypeError`, it is not caught by
the try/catch blocks, and can result in Flask-Restful returning a 400 status
code response.

This commit changes the default type to a wrapper around `six.text_type` that
only takes one parameter. This way, an encoding will never be specified, and it
will never raise a `LookupError`.
Fix typo
Adjust factory method to follow Flask extension guidelines

Based on 85cdf6bbb968debfa622ccb819ee9849b5a94e88 and
21c4ca7968c5da2c8d9fdb7f13774ec50fe062a5, authored by Matt Wright
<matt@nobien.net> @mattupstate
Fix for allowing redirects through to user
Don't use two checks; Just check the base class
Fix traceback being lost
Update Make targets; add deps to setup.py
Use a slightly more common test framework

This brings FR in line with the rest of Twilio's projects in terms of
installing and running the tests. Please let me know if this causes issues.

You can now run the tests with:

    make clean install test-install test

which will do a clean install and test. The next time you run the tests you can
simply use "make test" to do the job.
Add positive type. Update natural type. Add tests
update travis.yml
update test, add blinker dep
need a script tag
just return
Add support for ISO8601 intervals to the types field
Merge branch 'master' into more-types
add more tests
Merge pull request #160 from singingwolfboy/typo-fix

Fix typo
Merge pull request #169 from twilio/dates

Add support for ISO8601 intervals to the types field
Merge pull request #159 from singingwolfboy/avoid-lookuperror

Avoid unicode LookupError
merge master