Tests of SASL utils and fixes of founded bugs

This commit adds tests of SASL utils and fix most of founded bugs:
* CharsetsTest
* HexConverterTest
* UsernamePasswordHashUtilTest
* **ByteStringBuilderTest** - fixed few little bugs
* **StringPrepTest** - fixed bug in chars browsing - codePointAt() has own solution for reading pairs of surrogates, charAt() has to be used here - this patch solve [ELY-46](https://issues.jboss.org/browse/ELY-46)

**Update:** Problems in directionality of characters resolved.
**Update:** Tests extended and should be now covering all utils - remarks welcome.