Make buffer_length dynamic

[longemen3000]

this allows resizing the buffer on the REPL. it is needed, for calling get_fluid_param_string("Water,"JSON") without erroring.

[barche]

Actually, I don't understand why buffer_length and message_buffer need to be globals, this would cause trouble if this is used from multiple threads.

[ibell]

I'm not a julia person, but I guess this is to improve the speed by avoiding reallocation of resources

[longemen3000]

i did a check on all fluids currently present on CoolProp, and the longest is Ammonia with a required buffer length of 30916. making it 32768 (2^15) to allow some leeway, but i suppose the JSON of a fluid is the longest possible string that CoolProp could return

[barche]

I made a copy at PR #34 since I failed to modify this one, some of the calls to length needed to be replaced with Base.length because length was also in the function argument list and took precedence.