Add support for Nested schemas

[rbw]

Will result in extra queries; previously queried nested schemas should be cached (lru_cache)

• [x] Nesting
• [x] Querying
• [x] Sub-requests with full response content
• [x] Documentation

[rbw]

Something like:

class AssignmentGroup(Schema):
    sys_id = fields.Text(is_primary=True)
    name = fields.Text()

class Incident(Schema):
    __location__ = "/api/now/table/incident"

    sys_id = fields.Text(is_primary=True)
    number = fields.Text()
    description = fields.Text()
    short_description = fields.Text()
    impact = fields.Numeric(pluck=Joined.DISPLAY_VALUE)
    priority = fields.Numeric(pluck=Joined.DISPLAY_VALUE)
    assignment_group = fields.Nested(AssignmentGroup)
    opened_at = fields.Datetime()

[rbw]

Added schema nesting support in #27

class AssignmentGroup(Schema):
    sys_id = fields.Text(is_primary=True)
    name = fields.Text()

class Incident(Schema):
    __location__ = "/api/now/table/incident"

    sys_id = fields.Text(is_primary=True)
    number = fields.Text()
    description = fields.Text()
    short_description = fields.Text()
    impact = fields.Numeric(pluck=Joined.DISPLAY_VALUE)
    priority = fields.Numeric(pluck=Joined.DISPLAY_VALUE)
    assignment_group = AssignmentGroup
    opened_at = fields.Datetime()