please use const char * when possible

[gratefulfrog]

Hi!
Thanks for the nice library.

You might want to update the member functions to use const char* for compatibility with the Arduino String class?
As per:

int encode(char *output, const char *input, int inputLength);
int decode(char * output, const char * input, int inputLength);
int decodedLength(const char * input, int inputLength);

corrected files are attached.
Thanks!
B
src.zip