Allow match_on to be passed as an argument to VCR

The README says you can do this:
 
```python
my_vcr = vcr.VCR(
    match_on = ['url', 'method']
)
```

Which wasn't quite the case in the current code hence this pull request.

I added a test for it but maybe a more direct test (without depending on the implementation of the method matching) would be better.

Also vcrpy is awesome! :)

Thanks!