Implement job control

- Add a job control module for spawning and controlling co-processes
- Add three vimscript functions for interfacing with the module
- Use dedicated header files for typedefs/structs in event/job modules
Add BACKERS.md

List of Neovim backers provided by Bountysource
Format backers list
vim-patch:7.4.181

Problem:    When using 'pastetoggle' the status lines are not updated. (Samuel
            Ferencik, Jan Christoph Ebersbach)
Solution:   Update the status lines. (Nobuhiro Takasaki)

https://code.google.com/p/vim/source/detail?r=cb5683bcde03796baa7e845fd9a2fcaec3383538
Remove #ifdef FEAT_WINDOWS from vim-patch:7.4.181
vim-patch:7.4.184

Problem:    match() does not work properly with a {count} argument.
Solution:   Compute the length once and update it.  Quit the loop when at the
            end. (Hirohito Higashi)

https://code.google.com/p/vim/source/detail?r=9ac2fc63501d3eff92446c03b2822b30b169db5a
Run clint.py in some files
Fix/add more files with to clint-files.txt
Define special key for asynchronous events

K_EVENT/KE_EVENT are used to signal any loop that reads user input(scattered
across normal.c edit.c , ex_getln.c and message.c) of asynchronous events that
were not initiated by the user.

Representing non-user asynchronous events as special keys has the following
advantages:

- We reuse the normal vim redrawing code. As far as the rest of the code in
  edit.c/normal.c is concerned, it's just the user pressing another key.
- Assume less about vim tolerance for "out-of-band" modifications to its
  internal state.
- We still have a very complex codebase and it's hard to predict what bugs may
  be introduced by these changes. With this we implement asynchronicity in a way
  that will be more "natural" to the editor and has less chance of causing
  unpredictable behavior.

As the code is refactored, we will be able to treat user input as an 'event
type' and not the other way around(With this we are treating arbitrary events as
a special case of user input).
Remove old comment from job.c
Remove donation value from backers.md
Sort backers.md
Add some missing backers and remove duplicates