[PATCH] Fix situation where base instance lives on domain subpath

[hwelch-fle]

Resolves issue #4 by storing the url path and adding it back to any paths passed to clean_url

>>> client = DirectusClient('https://my.domain.com/X/')
>>> client.subpath
'X' # strip all leading and trailing slashes
>>> client.url
'https://my.domain.com'

>>> client.clean_url(client.url, '/articles/1')
'https://my.domain.com/X/articles/1'