implemented NetwellResponse

[Smarties89]

I have implemented NetwellResponse, that makes it simpler to assert contains, contains not etc.
(This is in response to #6 )
I implemented URl('...').response as a method as we discussed.
But I realised that it breaks backward compatibility (you can't use URL('...').response anymore.
Should I change it to be a simple property like before but with the included contains, ... methods?

PS.
To make life easier for you I included the script I tested it with:

from netwell.checkers import URL


# Should be ok
URL('https://www.gnu.org/licenses/gpl.txt').response().contains('GNU')

# Should return error
URL('https://www.gnu.org/licenses/gpl.txt').response().contains('Windows rocks')

# Should be ok
URL('https://www.gnu.org/licenses/gpl.txt').response().not_contains('Windows rocks')

# Should return error
URL('https://www.gnu.org/licenses/gpl.txt').response().not_contains('GNU')


def succeed_check(response, outcome):
    data = response.text
    if 'GNU' not in data:
        outcome.fail('Other data expected')


URL('https://www.gnu.org/licenses/gpl.txt').response().check(succeed_check)

[Smarties89]

Implementing response as a method instead of a simple wrapper created far reaching backward incompatibilities. Every custom check would stop working. Therefore I hope you came to the same resolution and also thought it would be a good idea to implement it as a simple wrapper.