[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::...
Merge branch 'issue58'
[client] Prepare for storing user configurations.
Merge branch 'issue58' of github.com:project-hatohol/hatohol into issue58