OPENDJ-1308 Migrate schema support

Added unit tests, simplified code and fixed one bug.

AbstractSubstringMatchingRuleImpl.java:
In DefaultSubstringAssertion.matches(), greatly simplified the code.
In rangeMatch(), forced byte comparison to fix a bug where overflowing was not happening due to int comparison being used.
Extracted BACKSLASH constant.

AbstractSubstringMatchingRuleImplTest.java: ADDED

SubstringMatchingRuleTest.java:
Improved the test code.

ApproximateMatchingRuleTest.java:
Fixed a comment.