clean up ValueConverter::convertType

[dbu]

check for TODO comments in https://github.com/phpcr/phpcr-utils/blob/master/tests/PHPCR/Tests/Util/ValueConverterTest.php, ensure full code coverage.

[mamazu]

Maybe some of the string related issues are also solved by #199

[dbu]

the thing about spaces not allowed in names was a misunderstanding i think. fixed it in #203 - the remaining TODOs are still relevant.