STM32F102 USB Programming
I have some bad news for you: the STM32F102 (and I suspect the STM32F1xx devices generally) don't have a USB bootloader, only a UART bootloader.  You need an STM32F2xx or STM32F4xx to get a USB bootloader loaded into the system ROM image.

You have many alternatives, including:


Use the UART bootloader, possibly with a USB-UART bridge IC like the FT232.
Provide your own USB bootloader that you load in via some other mechanism.
Use a JTAG/SWD programmer like the STLinkV2 (they're really cheap, but quite slow as far as JTAG goes).  This is your best option because it means you can debug when coding.