Improve yaml documentacion Fix #100

Extrtacted from https://github.com/schmittjoh/serializer/blob/master/tests/JMS/Serializer/Tests/Metadata/Driver/yml/ObjectWithVirtualProperties.yml
Allow to fetch data from has*() with public_method

Same as is*() method, has*() is mostly used with booleans
tests for accessor (public_method)
Added test for leading backslash in front of class name to TypeParserTest

testLeadingBackslash added to check if there's a leading backslash in
front of a class name.

Signed-off-by: Alexander Kluth <contact@alexanderkluth.com>
Add support for XML namespace on the XML root element
Changed some constraint to allow latest versions
moves driver creation to dedicated class (closes #247)
fixes a regression
Fixed yml mapping file name
adds a test for handling single table inheritance with proxies