String: Added four new methods to string.nit

Four new methods have been added to abstract class Text : is_upper, is_lower, to_snake_case, to_camel_case. The latter two relies on the two others in their implementation. Converting case may be useful for code generation, especially when wrapping extern classes in order to conform to snake case nit standard. 
Expliquer la regle puis mette l'exemple du _ prefixe