Schematic

    I've tried to choose smallest components I could find on the market. This is the result.

    I think an Attiny84 was a good choise. It's a low cost MCU, has some USB support (software USB) and of course it's quite small. Originally I wanted to use an Attiny85, but it has only 6 I/O's.

   This is a standard USB schematic for the V-USB. To make it smaller and to cut down the cost I've used a PCB USB connector.

   I wanted to use the original idea(PICKIT 2) for boosting the VPP, but I couldn't afford to use 2 pins to control it. So a TPS61040 was chosen. I could use a lower cost booster, but unfortunately they are bigger.  Another advantage of this booster is that it runs at around 1MHz, which means that filtering components are smaller.

   I was very happy to find PUMD13. It has 2 transistors (NPN and PNP) and some resistors. So I have replaced 4 transistors and 8 resistors with just 2 of this devices.

    And two LED's for better user experience. One is for USB power. It was hard to decide where to connect the second LED. I've decided to connect it to the target VDD. 

   And a few capacitors to filter noise and other stuff.

  Full schematic can be downloaded here.