move argparseui.py into a package to fix namespace problem

With argparseui 0.0.2 (installed with ``pip install -U argparseui``)

```python
In [1]: import argparseui

In [2]: dir(argparseui)
Out[2]: ['__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__']
```
Note that the ``ArgparseUi`` class is not shown in the output of ``dir``. You would have to type ``import argparseui.argparseui`` to see the Ui class.

with this PR (and using ``pip install -U -e .`` in the git repo)

```python
In [1]: import argparseui

In [2]: dir(argparseui)
Out[2]: 
['ArgparseUi',
 '__builtins__',
 '__doc__',
 '__file__',
 '__name__',
 '__package__',
 '__path__',
 'ui']
```