Use company-mode for completion

- remove packages related to smart-tab, not needed anymore
- create a basic configuration
Spacing corrections
Define tab to complete always, except listed

Define a function which handles the TAB key for company mode:

 1. if in the minibuffer do completion there the normal way
 2. if looking at the end of a symbol, complete it with company mode
 3. else indent according to mode.

Make exceptions for:
 help mode
 magit-status-mode

(Probably a lot more to follow later on)

Also, not sure if looking at the end of a symbol is always the right way
to go, but it's a start.