Add SetText() as analog to GetText().
Add comment for SetText().
Make SetText() behave like the docs say.
Add tests for SetText().
trim the test cases and review
add variants of SetText() to support types
Merge branch 'master' of https://github.com/leethomason/tinyxml2 into setboolfirstchild

Conflicts:
	tinyxml2.cpp
	tinyxml2.h