Added and updated API which was changed from old TStr but still fits well with new TStr.
mutable methods
Merge pull request #115 from blazf/TStr

T str
Merge branch 'TStr' of https://github.com/qminer/qminer into TStr

Conflicts:
	src/glib/base/dt.cpp
	src/glib/base/dt.h
Merge pull request #116 from rupnikj/TStr

T str
...
Merge branch 'TStr' of https://github.com/qminer/qminer into TStr