Fix some minor spelling and grammar in documentation
Merge pull request #684 from brianhelba/doc-spelling

Fix some minor spelling and grammar in documentation
Corrected Travis config syntax
Ensure that the id_field is always added to field mappings

Previously, TopLevelDocumentMetaclass was only adding the default 'id'
field to '_db_field_map' and '_reverse_db_field_map' for Document classes
that are derived from another non-abstract Document class. This commit
ensures that 'id' is available in these two dicts for all classes.
Ensure that 'id' is present and first in '_fields_ordered' for all classes

An 'id' field only needs to be explicitly added to '_fields_ordered' by
the TopLevelDocumentMetaclass if 'id_field' is not provided and the 'id'
field was created in the metaclass. Otherwise, the field would have been
added to '_fields_ordered' along with the other user-defined fields. Thus,
the conditional for explicitly adding 'id' to '_fields_ordered' can be
moved inwards in scope.

More importantly, 'id' should always be the first element of
'_fields_ordered'. This may not always occur if a Document class is
derived from a non-abstract Document class. This commit ensures that 'id'
will remain the first element for all classes.