Added the ability to use a custom `tempdir`

Currently, to change the `TMPDIR`, I must start `pserve` with:
```
TMPDIR=/home/raj/py2_projects/newsite/newsite/_temp pserve development.ini
```

Instead, I'd like to be able to pass the desired `tempdir` in the view code. Amazingly, you already had manager.py setup to accept the `tempdir` argument, being that standard library's `tempfile` accepts it as well.
Giving `create_file` a `tempdir` argument

Needed to put a `tempdir` argument in `create_file`
Added test for `tempdir` argument on `create_file`
Updated docs to note that create_file now has a tempdir parameter

Modified docs to reflect that `create_file` now accepts a third parameter `tempdir` for specifying a custom temporary directory for `tempfile` to use, rather than the environment variable.
Using tempfile.mkdtmp instead of os.getcwd

Removed explicit directory creation within current working directory.
Don't capture stdout during tests.
Test the changed module API function, not the unchanged method.