Change prefix binding for Spell commands to `S`

Instead of `s` previously
Change auto-highlight-symbol prefix binding to `s`

`s` is now the mnemonic for `symbol`
previously it was `h`
Add `sl` to list symbols in a buffer

Using `helm-semantics-or-ido`
Update readme with last modifications

Prefix change for Auto-highlight-symbol
Prefix change for Spell commands
Add a binding to list the symbols of a buffer