555 Temperature Controlled PWM
You can accomplish temperature feedback by connecting the analog voltage out pin of the TMP36* to pin 5 of the 555. A temperature increase will increase the voltage at pin 5 of the 555, this increased voltage will increase the "on" time of the 555 as I've outlined elsewhere. This will, in turn, increase the voltage that the fan is experiencing and thus its speed! You will need to tune the size of the potentiometer and capacitor to suit the application.

*Connect the other two pins to \$V_\text{cc}\$ and ground according to the datasheet.