Add simple Hotkey Mode functionality

This adds a simple mode.lua library to Hydra.

Example usage, using the grid.lua library:

```lua
moom = ext.mode.bind({'shift', 'cmd'}, 'M',
   function() hydra.alert('Move Windows', 3) end,
   function() end)

moom:bind({}, 'ESCAPE', function() moom:exit() end)
moom:bind({}, 'w', ext.grid.pushwindow_up)
moom:bind({}, 'z', ext.grid.pushwindow_down)
moom:bind({}, 'a', ext.grid.pushwindow_left)
moom:bind({}, 's', ext.grid.pushwindow_right)
moom:bind({}, 'm', ext.grid.maximize_window)
```

Closes #138
Update version numbers.
Better syntax-error handling in default IPC handler.
switch example config link to raw github
Merge pull request #211 from benfb/master

switch example config link to raw github
Update sample config link.
Update sample config link again.
Closes #210.
Pulling hydra-cli into Xcode project; goodbye github.com/sdegutis/hydra-cli :'(
Updating hydra-cli docs.
Extracting IPC module.
Bumping version so I don't forget later.
Renaming hydra-cli executable product and copying it to Resources instead.
Updating installation instructions.
Renaming binary in man-page.
Adding ipc.link and ipc.unlink
Adding IPC installation steps to README.
Saying "done" somewhere in the code.
Updating IPC installation instructions.
Update IPC stuff.