Create SQLAlchemyConnectionField through a factory method.

[palmkevin]

Overriding the constructor-method gives the user the possibility to enhance the standard implementation of connection fields. (If you want to add filtering/sorting for example)

[coveralls]

Coverage decreased (-0.2%) to 90.083% when pulling e5bdaec on palmkevin:ConnectionFactory into c07b746 on graphql-python:master.

[coveralls]

Coverage decreased (-0.2%) to 90.083% when pulling e5bdaec on palmkevin:ConnectionFactory into c07b746 on graphql-python:master.

[coveralls]

Coverage decreased (-0.2%) to 90.083% when pulling cbf0a22 on palmkevin:ConnectionFactory into c07b746 on graphql-python:master.

[palmkevin]

I closed this pull request because it is duplicated by another pull request that has better code coverage
#46
The implementation of the other pull request is the same than mine (I am member of the labsolutionlu-organization)