Add method in StatModel class, to load from String

Added a method "loadFromString" which is based on the "load" one. It
allow to directly pass the XML string which can be usefull and faster
when you have a huge file in a variable.
Changing tabs to spaces.

Edited previous code to change tabs into spaces.
Re-editing Documentation

Attempting to re-editing the documentation, to satisfy buildbot error
status.
Re-edition of documentation

Re-edited the documentation again.
Removed "FORMAT_XML" to generalize format

Removed the "FileStorage::FORMAT_XML" to generalize format of the String
model.
Merge pull request #3473 from TobyWanKenobi:master