New Feature - Simplified field lookup type filters including tests and docs

RDF/django-filter currently supports filter_fields for exact filtering only:
```
class ProductList(generics.ListAPIView):
    queryset = Product.objects.all()
    serializer_class = ProductSerializer
    filter_fields = ('category', 'in_stock')
```

Creating many custom lookup filters can be tedious with DRF and django-filter:

    class SessionFilter(django_filters.FilterSet):
        client_type__contains = django_filters.CharFilter(name='client_type', lookup_type='contains')
        client_system__contains = django_filters.CharFilter(name='client_system', lookup_type='contains')
    
        starttime__gte = django_filters.DateTimeFilter(name='starttime', lookup_type='gte')
        starttime__lte = django_filters.DateTimeFilter(name='starttime', lookup_type='lte')
        class Meta:
            model = Session
            fields = ['client_type',
                      'client_system',
                      'starttime',
                      'project',
                      'language',
                      ]

    class SessionList(generics.ListAPIView):
        queryset = Session.objects.all()
        serializer_class = SessionSerializer
        filter_class = SessionFilter

Here's the same thing with TastyPie:

    class SessionResource(ModelResource):
        class Meta:
            queryset = Session.objects.all()
            resource_name = "session"
            allowed_methods = ['get']
            serializer = JSONSerializer()
            filtering = {
                "client_type": ['exact', 'contains',],
                "client_system": ['exact', 'contains'],
                "starttime": ['exact', 'gte', 'lte'],
                "project": ['exact'],
                "language": ['exact'],
            }

I've implemented support for a fields Dictionary for django-filter. This Pull request includes tests, docs, and is backwards compatible for existing implementations.
 
    import django_filters

    class ProductFilter(django_filters.FilterSet):
        class Meta:
            model = Product
            fields = {'name': ['exact', 'icontains'],
                      'price': ['exact', 'gte', 'lte'],
                     }

The filters will be available as 'name', 'name__icontains', 'price', 'price__gte', and 'price__lte' in the above example.

This feature request was first discussed on the [DRF issue queue](https://github.com/tomchristie/django-rest-framework/issues/1856), but I realized it could be entirely implemented in django-filter.