properly deal with case insensitivity
expose retrieve methods that take ETFilter instead of filter string
(getFilterQueryParams): Added support for "and" expressions.
(getFilterQueryParams): Throw an exception when an "or" expression
is supplied (the Audience Builder routes don't support them).
(getFilterQueryParams): Convert " " to "%20" in property names and
values.
(getFilterQueryParams): Ensure property is not null before calling
getInternalProperty().
(toQueryParameters): Throw an exception if an operator is unsupported.
(retrieve): Added support for sorting via orderby strings.