Allow indention string to be customized

With this patch, the indention string of the generated PHP code can be customized, allowing to increase or decrese the indention level. This can be useful if you want the generated code to be compliant to a specific coding standard. For example, PSR-2 requires that the indention level is 4 spaces, but the standard indention used by Wsdl2PhpGenerator is hardcoded to a 2 spaces string. By setting `$indentionString = '    '` as `Config` class constructor argument, the code will be generated using that custom string instead.