Makefile: Build cleanly on OpenBSD

Thanks to jca at wxcvbn dot org and Github user @clod89 for the
providing the patch.

Also see bug #17
Reformatted the README file
Unified subdirectory handling with the file monitors

Previously, monitors were installed for each (sub-)directory specified
on the command line, but not for new directories detected by a
previously installed monitor. Directory monitors do not work reliably
with sub directories, therefore I changed the code to always setup a new
directory monitor.
Release preparations: Updated configure script with new MANDIR capabilities and removed update notice from 1.0 → 2.0
Release preparations: Do not fail for obsolete options yet, only show a warning
Release preparations: Make make install target GNU compliant

i.e. do not strip, ignore failed man-page installations
Fix image order for unsorted display

See bug #18. In unsorted mode, I sort the tree by integer comparison,
where the index comes just from incrementing a counter. The
compare-function was <=, when > would have been correct.

This also blocks bug #16 (2.1 release) for another few days.
Show an error message if --shuffle and --sort are combined
Readme: Added information on multi-head problems
README: Clarified the randr problem
Linked my fakexrandr into the Readme
Bugfix: Redraw image upon scale factor changes
Draw the info-box smaller if the window is too narrow to fit
Move image unloading to main thread to avoid race condition
libbostree update

Pulled because there was a bug in the remove method
Update pqiv version to 2.1
Merge branch 'master' into redraw