Added new keywords to String library: Convert To Lowercase and Convert To Uppercase
Modified based on the comments
Added note to AUTHORS.txt
Combined test suites, should hava done that in the first place
+ Fixed AUTHORS.txt to contain name only one time
+ Added empty line at the end of the test data
Merge pull request #1744 from aaltat/master

Added new keywords to String library: Convert To Lowercase and Convert To Uppercase