Allow classes, functions, exceptions to be conveniently imported from the package.

[mainanick]

The PR allows package level import

from pymemcache import Client, PooledClient, MemcacheError, ...

Currently its

from pymemcache.client.base import Client, PooledClient
or 
from pymemcache.client import Client, MemcacheError

[mainanick]

Tests seems to fail with a "weird" error
Command "python setup.py egg_info" failed with error code 1. ImportError: No module named six

[nichochar]

This is a nice quality of life change, I'll look into the six error.

Six is a package to help make python2 and python3 versions compatible, I wonder why it's complaining on this seemingly unrelated change.

Six is specified as a dependency here

I wonder if something happened at the Travis level which lead to this

[nichochar]

Thanks