DATACMNS-402 - Add support for sorting by a QueryDSL OrderSpecifier.

Previously we did only support ordering by a string or property path through Sort. In order to be able to express more type safe ordering expressions we introduce QSort as a subclass of Sort that is able to wrap QueryDSL OrderSpecifiers. To be able to separate the concerns of sorting via QueryDSL expressions better we introduce the AbstractPageRequest base class and QPageRequest as a special implementation that accepts a QSort or OrderSpecifiers for ordering.

Original pull request: #59.
DATACMNS-402 - Slight refinements in QSort implementation.

Re-enable concatenation with plain Sort via ….and(…) method. Some general polishing and JavaDoc cleanups.

Original pull request: #59.