Measuring small (and small changes in) capacitance
Another way to detect such capacitance changes is to build the capacitor into an L-C oscillator (such as from a radio) and measure its frequency. RF oscillators are usually better than 555 timers if the capacitance is much below 100pf. 

You may have to compensate for temperature and other causes of frequency drift but these are usually relatively slow compared to the effects you want to measure.