Remove hardcoded prefix /usr/local/

Otherwise you cannot install it in a virtualenv
Add support for extra button
Fix a possible deadlock

gauge_close will hang if the process spawns a new child after
gauge_start and the child does not terminate before gauge_stop. This
cannot be fixed for all cases. When forking, the child process should
close all open file descriptors that it does not need, but there is
a way to automatically close an fd if an exec() is executed.
Add support for progressbox dialog
Merge https://github.com/skalkoto/pythondialog