Device support: PL2303.

This change consolidates changes made by Felix Hädicke:
  https://code.google.com/r/felixhaedicke-usb-serial-for-android/

Upstream changes merged (newest first):
  3b6fb7f Rename parameter variables
  74d858f Remove unused constant
  bab0691 Make consts private
  381f28c Remove test code
  bdd9a64 Suppport for reading CTS / DSR / CD / RI status
  ccf807a Remove "Untested" TODO comment (no longer true)
  1b1ccce Remove unneeded commented out function ctrlIn
  5f853db Make parameter fields private
  d8c6758 Fix for wrong constant being used when turning off DTR
  05ff566 Fix for NullPointerException
  9d2c9af Reenabled device type detection
  307ce72 Device support: PL2303.

Original commit message:
  New driver for Prolific PL2303

  A new driver for Prolific PL2303 devices, which is more or less a port
  of the "pyprolific" driver from https://github.com/eblot/pyftdi
Support flushing non-written / non-read data

* Add a new flush(flushRX, flushTX) function to the UsbSerialDriver
interface
* Add a default flush implementationt to UsbSerialDriver which always
returns false
* Add a real flush implementation to the FTDI driver
* Adjustment of the existing Prolific driver flush function to implement
the UsbSerialDriver flush function