Examples: fix models, extend Flask example

[avbentem]

This PR only applies to the example code.

• Small fix: the example models used the wrong name for a backref, resulting in role.roles rather than role.employees.

• Small fix: in https://github.com/graphql-python/graphene-sqlalchemy/pull/210/files#diff-b14ba1019bb9939dd94d33195e63ae48L35-L40 the example for a single sort column was replaced by sort=Employee.sort_argument(), which is not the same. (It allows for sorting on multiple columns simultaneously, just like when not specifying sort= at all.) While fixing and documenting that: as all_employees has more columns than all_roles, it makes sense to allow multiple sort columns for the first, and only one for the latter.

• Added an example totalCount attribute for the connections in the Flask example, as per How to have edges/totalCount on a relationship field? #58.

[coveralls]

Coverage remained the same at 97.611% when pulling 4524a44 on avbentem:master into 20ecaea on graphql-python:master.