Tests, fix and docs.

Hi Teddy,

Long story short, I've got:
1. Made unit tests. Will just copy related comments on test run and scope from source code:

```python
# Values for test described in APIv3 class definition below.
# To run test cases go to the directory with current file and run:
# $ python __init__.py
TEST_SLUG = 'dpd-uk'
TEST_TRACKING_NUMBER = '15502370264989N'
TEST_API_KEY = 'YOUR_API_KEY'
    ...
"""
Test code goes below.

Test covers all accessing methods (POST, GET, PUT, DELETE).
Test covers all variants of building specific API calls (endpoints paths + body):
   ...
Test checks conversion of input list type parameters to comma separated strings.
Test checks conversion of input timestamp strings to datetime variables.
Test checks conversion of output timestamp strings to datetime variables.
    ...
"""
```
2. Thx to tests, I've found a bug that brakes API endpoint path creation. Fixed.
3. Made user documentation: https://github.com/korshunov/aftership-python/blob/master/README.rst

Two questions:
1. Are there any changes in API v4 that breaks calling conventions? If no, I'll just change **api.aftership.com/v3/** to **api.aftership.com/v4/** for new API support.
2. Are you happy with changes? If yes I'd like your approve to put this module to PyPi (official Python modules storage). After that, users would be able to install SDK in one-line via pip command (showed in README).

Thanks in advance!
Fedor