PWM module bugfix and clarification.

1) PWM write function used incorrect (constant) period value to
calculate active time.
2) Clarified internal data structure by changing it from list to dict.
Adde user object support for interrupts
Removed a leftover variable from ADC module
New basic examples

Seven different example programs, which show how to use the basic
functionality: GPIO read/write, interrupts, PWM, ADC.