bug 28643 improvement to serbian variants conversion

bug 28643 improvement to serbian variants conversion

This patch is a PARTIAL merge of /branches/nikola/phase3 ::

r85224 avoid double conversion when text already use the correct variant
r85239 minor fixes to previous
r85308 documentation (@since 1.18 update to 1.19)
r101359 guessVariant doc + boolean typecast
r101369 tests
r103131 additional test

Test plan:
==========
$ ./phpunit.php --filter LanguageSr
PHPUnit 3.6.3 by Sebastian Bergmann.

Configuration read from /srv/trunk/tests/phpunit/suite.xml

.....

Time: 1 second, Memory: 78.50Mb

OK (5 tests, 19 assertions)
$