Improvement: Change VSync mode with D-Bus & Makefile update & Misc

- Add on-the-fly VSync option modification via D-Bus, as requested by
  kunitoki (#80). Expose parse_vsync(), create vsync_init() and
  ensure_glx_context().

- Change default value of ps->drm_fd to -1.

- Update Makefile. Change the install/uninstall rules and add doc
  installation, requested by hasufell in #85.

- Mark window not damaged in map_win(). It helps in reducing flickering
  with inverted window color, but I'm not completely sure if it's safe.

- Avoid modifying w->invert_color when window is unmapped.

- Update documentation. Thanks to hasufell for pointing out.

README.md

@@ -50,7 +50,7 @@ __R__ for runtime
 * pkg-config (B)
 * make (B)
 * xproto / x11proto (B)
-* bash (R)
+* sh (R)
 * xprop,xwininfo / x11-utils (R)
 * libpcre (B,R) (Can be disabled with `NO_REGEX_PCRE` at compile time)
 * libconfig (B,R) (Can be disabled with `NO_LIBCONFIG` at compile time)