[mlpl] Rename va_list version of StringUtils::sprintf() to StringUtils::vsprintf().

Because many compile time warnings are occured on my environment:

  ActionManager.cc:190:3: warning: deprecated conversion from string constant to 'va_list {aka char*}' [-Wwrite-strings]

and they cause crash at runtime.
Merge pull request #76 from ashie/vsprintf

[mlpl] Rename va_list version of StringUtils::sprintf() to StringUtils::...