Fixes #41 cannot insert `f` in ex command mode

One caveats, the waiting timer for `fd` sequence seems to
be overridden somewhere so to press `fd` you have to wait
2 seconds between the `f` and `d`, fortunately this
sequence should not occur very often.
Fixes #44 error "subword-mode is not currently registered as a minor mode"
Remove annoying and unneeded yasnippet loading on idle
Change default aspell dictionary to english instead of en (not working on windows with cygwin aspell)
Fixes #45 maximize frame if dotspacemas-fullscreen-at-startup is t

Also add toggles for both frame maximization and fullscreen.
Can pass font options in spacemacs/set-font
Disable EDTS on Windows (not officialy supported for now)
Add vi-tilde-fringe extension
Update screenshot
Add magit-gitflow and fix the `f` issue for feature

The solution to fix the `f` binding issue is to not
alter emacs-state-map. By keeping emacs map untouched
we have a fallback for modes like magit-gitflow which
require the `f` key.

Indeed with the current implementation of the `fd`
sequence it is not possible to guess what the `f` key
must do for a given mode. The only solution for now is
to keep one not modified state map (emacs is a good candidate
for this).
vi-tilde-fringe is now available in MELPA

Also hide the lighter.
Add screenshot of spacemacs in urxvt
Add terminal screenshot in readme
Add git-flow instruction for contributions in readme
Set `C-u` to scroll-up and `<SPC> u` to universal-argument
Update old name of submodules in .gitmodules
Lazy loading of tern-auto-complete
Lazy loading of helm
Fixes #46 Weird behavior of projectile

Projectile takes time to load (around 0.2 seconds) and it is
lazy loading friendly if we want to enable it globally.

To be able to lazy load it, we encapsulate the binded calls
in spacemacs/xxx functions which take care of firing the
mode globally if required.
Correctly load helm bindings at initialization time

instead of at loading time which is useless :-)