[PATCH python-hglib v2] Add feature to allow hglib user to get call backs for prompts, output

[Barry Scott]

FYI: My code that uses this patch is in SCM Workbench.

Here is the code I use for init() and clone():

https://github.com/barry-scott/scm-workbench/blob/master/Source/Hg/
wb_hg_project.py#L65

Here is the object that handles the call back interface and map into my GUI's 
world:

https://github.com/barry-scott/scm-workbench/blob/master/Source/Hg/
wb_hg_project.py#L448

Barry