Don't clobber system build flags

Currently bspwm clobbers system build flags. This prevents
stack-smashing protection and other compiler features from being
applied.
Merge pull request #26 from Stebalien/dont-clobber-build-flags

Don't clobber system build flags
New message: `fence_ratio`
Use empty sequence element in example sxhkdrc
New setting: `history_aware_focus`
Revert bogus push/pull factorisation
Generate documentation via `pandoc`
Place build instructions in separate file
Honor `history_aware_focus` for `shift`
Update source dependencies
Refresh TODO list
Rename textwidth to txtw
Generate documentation via asciidoc
Four hyphens is enough
Move documentation into doc/
Handle insertion at non focused nodes
Make split mode and direction local
New option for `cancel`: --all
When shifting into manual mode nodes: transplant
Skip non tiled windows in `find_biggest`