Features, Installation and updates, Configuration and typos

Streamline descriptions and remove some lists of specific features. Also
reorder headings to focus on features with lists and pretty pictures to
make things more attractive.

I felt that drawing attention to specific Emacs packages in the readme
would be meaningless to people coming from Vim. They're covered in the
main documentation file anyway.

Properly define what a configuration layer is, and describe the main
roles of the ~/.spacemacs file.

Provide a brief overview on how to load config layers, since this is the
something users will probably want to do immediately.

Fix typos: Identified in review of c36a36fecf7
Fix missing `~` in README and missing Paradox link in DOC
Clarify adding package in configuration layer
Skip installation of nonexistent packages

When attempting to install a package on startup, test whether it is in
the package archives list. If it is not, skip the installation attempt
and issue a warning.

Resolves #212
Integrate ahs with evil search functionality

When auto-highlight-symbol is used, it does not currently place the
searched term into the search ring, so that after ahs mode is left,
hitting "n" or "N" does not return to the search term that was selected
by hitting "*" or "#".  Some extra steps must be taken to perform the
expected search behavior of "n" and "N" after "*" or "#".

This also integrates the searched term into the ex mode substitute
command, so that "%s//replace/" works as expected.
Remove Shift+Return to exit iedit-insert state

It is not consistent with the rest of spacemacs
Enable evil-escape `fd` in iedit states to exit to `normal state`
Update iedit layer readme
Activate leader in iedit and iedit-insert states
Change color of iedit states
Completely changed the philosophy of iedit state

It is now a stock normal state adapted to iedit
Add compilation support

* spc c c helm-make-projectile
* spc c C compile
* spc c r recompile
Free up `d` in iedit state and `D` now deletes all occurrences

ALso reformat the README
Remove unused <SPC> c r to comment region and prevent conflict

with compilation commands.

<SPC> ; does the job of <SPC> c r.
Add tip for MacType to get smooth fonts on Windows
Add a section for shells in documentation
Add FAQ questions to readme and <SPC> f e d to open the dotfile