Added default port if ANDROID_SERIAL contains only IP address

- Version 2.3.4
- Avoid checking adb's executable permission on Windows (as it was
reported as failing by several users)
- Remove environment when using adb to obtain serial number of the
device because if the serial number was specified as an environment
variable and also contains a regular expression adb will always return
'unknown'
- Added ADB_DEFAULT_PORT
- Avoid trying to convert version property to int because it's None on
some devices