Add const to member functions in StringUtil and its derived classes

In particular, explicitly adding const to StringUtil::showString would be
important because it could be used a lot by clients when they want to get
a std::string object from the internal representation of KyTea's string.

Also, add comments regarding the thread safety to mapChar and mapString.
Merge pull request #11 from tetsuok/add-const-to-stringutil

Add const to member functions in StringUtil and its derived classes