Updated StringUtils and StringUtilsTest.

Added methods strip(), isSecure() and isValidIsbn13() in StringUtils.
Added testcases in StringUtilsTest.
Testcases for isBlank() created.
Testcases for join() created.
Testcases for isValidISBN10() added.
Removed unnessecery method isEmpty() -> isBlank() is the method to use.
Testcommit to check ssh-key. No change of source.
Fixed warnings in StringUtils and StringUtilsTest.