Add CWOP upload support

I've added support to pywws to upload to the APRSWXNET network to contribute to the CWOP (http://www.wxqa.com/).
Since pywws was supporting only HTTP calls, I had to include some changes to allow to send other kind of packets, but I tried to be less invasive that I could.

To make this feature possible, I've also introduced some minor changes that should be quite generic:
- Added "altitude", "latitude", "longitude" parameters to wheater.ini generic
  section (since these won't change per each service I think it's a good thing
  to store them only in one place, and eventually share them between services).
  These are stored in the local_data dict that can be accessed by templates or
  directly or using the "#key" directive.
- Added function to compute rain in 24hrs
- Added conversion functions to convert meters to feet, coordinates to LORAN
  format and to limit the length of an integer (in terms of printable length).
- A service can now define a "minwait" configuration option that is used
  to define the number of seconds that must elapse between two dispatches.