If you are interested I have posted an initial version of the implementation of a Signal Capture library for the mbed/mBuino platform.
It supports the capture of digital signals using either polling via the DigitalPinSignalCapture class or on interrupt enabled pins using the InterruptPinSignalCapture class. The interface between the two is exactly the same, so they can be used interchangeably depending on your requirements and availability of interrupt pins on your target mbed platform.