Added herbstluftwm-tutorial files to .gitignore

This adds herbstluftwm-tutorial.7 and herbstluftwm-tutorial.html to .gitignore,
just like the other generated docs already are.
Add modeline ft=asciidoc to docs
Add an 'uninstall' target to the Makefile
Use "output" for all IPC commands, not "result"
Remove the ";" at the end of the HSDebug macro
Fix whitespace/comment
Add glib backports for glib 2.14 systems

glib-backports.h implements glib functions that are not available in old
glib versions. This lowers the minimum required glib version to 2.14
(tested on CentOS 5.5).
Fix argument checking for merge_tag completion
Default to "toggle" for on|off|toggle commands

For "fullscreen" it already was in the manpage, but not in the code.
Use HERBST_NEED_MORE_ARGS if a cmd needs more args

Now a command returns HERBST_NEED_MORE_ARGS in the specific case when it needs
more arguments, instead of HERBST_INVALID_ARGUMENT. This allows ipc-client to
output an error message in this case.
client: Print \n to the same file as the output

If g_ensure_newline is enabled then print the newline to the same file
(stdout or stderr)as the actual output of the command.
Output error message if command is not found
Add error output for nearly every command
Check if the rule command has enough arguments
Return status in key_remove_bind_with_keysym
Add error message when an unbound key is unbinded
split: print error message if alignment is invalid
Print error when frame neighbours are missing
bring: add error message when client is invalid
Make arguments mandatory in jumpto/bring

When jumpto/bring were called without an argument, they defaulted to "currently
focused client", but that doesn't make sense with these commands.