Merge lp://staging/~piastucki/bzr-xmloutput/progress-monitor into lp://staging/bzr-xmloutput

This branch adds support for simple progress reporting.
Some bzr commands like checkout or branch can take a lot of time and there is no way to report the progress when invoking them via xmlrpc currently. The patch adds the following changes:
1) when starting XMLRPS server, the code will check if BZR_PROGRESS_BAR variable equals to 'xmlrpc' and start up an additional XMLRPC server at port+1 in this case. The additional XMLRPC server is needed to get the progress of the command while the main XMLRPC server can still remain single-threaded.
2) UIFactory adds a custom progress view implementation (XmlRpcProgressView) that stores the current progress in a map
3) The additional XMLRPC server exposes a single function called 'get_bzr_progress' that gets the progress based on the command passed from the map
4) to avoid the need to pass an additional command ID to lookup the progress for the desired command a hash code based on command arguments is used as the key instead.

There are some test cases included, moreover, the implementation is also successfully used in bzr-java-lib and bzr-eclipse (https://launchpadlibrarian.net/138446785/2013-04-26%2011-03-52.png)

The functionality seems to be very specific to XMLRPC server interactions and if you feel it is out of scope of bzr-xmloutput feel free to reject the merge proposal.