Added support for float values for experience stages and skill/magic rates.

Closes #605
Added support for float values for experience stages and skill/magic rates. Adding new float-valued configuration variables should be quite easy now.
New functions:
ConfigManager::getGlobalFloat(...)
ConfigManager::getFloat(...)

I'm wondering if the ConfigManager::getGlobalNumber(...) and ConfigManager::getNumber(...) should be renamed to ConfigManager::getGlobalInteger(...) and ConfigManager::getInteger(...) respectively for better naming consistency.