Topic/const correctness

a)
Functions receiving strings as arguments usually don't modify this argument, so they should be declared as 'const char *' such that one can pass either a 'char *' or a 'const char *' when calling the function.

b)
Functions which return a string usually return a statically allocated string that the user is not allowed to modify/free. Better declare the returned string as 'const char *'. This is also necessary if the function may return one of its argument which is const, such at dictionary_get() when it returns the default value.

c)
Removed a few trailing white spaces