Implement droplets

I added the Client.droplets function which returns a list of Droplet objects.

Additionally I modified the method of the `_version__` local variable. The `from drop.client import Client` threw an error when installed from source using `python setup.py install`. I think it's a better approach.

The error was:

```bash
(venv)☁  ~  python
Python 2.6.8 (unknown, Aug 25 2013, 00:04:29)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from dop.client import Client
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/apapai/venv/lib/python2.6/site-packages/dop-1.6.b2-py2.6.egg/dop/client.py", line 15, in <module>
    import __version__
ImportError: No module named __version__
```