Added temperature converter example

A couple differences exist between this and the code in K&R. I added a return value on main, as required by the C99 standard:

> **(5.1.2.2.1)** It shall be defined with a return type of int and with no parameters ... or with two parameters ... or in some other implementation-defined manner

Also added the void parameter on main. Empty parameter list means the function takes an "unspecified" number of arguments. Placing `void` as a parameter explicitly specifies zero parameters.