New variable `dotspacemacs-mode-line-unicode-symbols`

Can choose between ASCII chars or unicode symbols in the mode-line
can be useful for some terminals
Put back golden-ratio on `tg` and move guide-key on `tk`

Also change the lighter of guide-key to K
Add more help describe key bindings
Update docs with more information on powerline colors

The powerline color issue is due to the fact that Emacs 24.4 now
defaults to using sRGB colorspace on OS X 10.7 and higher, not some
"brokeness" with brew's `emacs` build as the previous verbiage seemed to
imply; the powerline codebase is still assuming `(setq
ns-use-srgb-colorspace nil)` is the default.

This PR adds more information about this so that users may
decide whether they want to just set it back to nil, switch to
emacs-mac (which has it set to nil anyway), or disable powershell
separators. (Turning off srgb colorspace results in washed-out theme
colors).

--------

Side rant:
I don't agree that the recommended fix for this would be to use
`emacs-mac-port` as that basically just gives better trackpad support.
IMHO trackpad support does not outweigh the frustrations over default
meta/super key bindings, no `server-start` support, or the use of the
antiquated carbon api, but I left the recommendation as-is in this PR.
Add link to osx layer in README
Fixes #331 On Windows SPC does not work in org files
Fixes #327 Add binding for window split + move to window
Documentation for `w V` and `w S`
Fix go to init function in helm-spacemacs

The function may fail when looking for an init function in
the current buffer.
Add powerline separators anti-aliasing FAQ
Use relative links in README whenever it is possible
Move layer images into img directories
Add note on emacs-mac-port server behavior in README
Fix duplicated battery info in mode-line
Improve battery text info and faces
bind SPC TAB to alternate-buffer
Minor table reformating in documentation
Minor change to battery info