Refactor install.php

- Remove all references to update functions
- Improve step2 by checking if password exists (form) or mail (persona)
- Add a param($key, $default=falsse) function to get $_POST vars
- Fix coding style (mainly spaces)
Merge branch 'dev' into 411-update-system
Add basic system of update

- Check on update.freshrss.org for new updates
- Download script
- Apply script
- Need translations and verifications

NOTE: current script on server indicates version 0.7.3 is an update
of 0.8-dev ==> IT'S ONLY FOR MY TESTS!

Script just does a backup of ./data actually...

See https://github.com/marienfressinaud/FreshRSS/issues/411
Add translations for update system
Improve update API

Update script must implement 4 functions:
- apply_update() to perform the update (most important). Return true if
  all is ok, else false.
- need_info_update() returns true if we need more info for update, else
  false. If this function always returns false, you don't need to
  implement following functions (but it's better to not forget)
- ask_info_update() should be a HTML form to ask infos. Method must be
  post and action must point to _url('update', 'apply') (or leave it
  blank)
- save_info_update() is called for POST requests (to save form from
  ask_info_update())
Litlle improvements (update system)

- Check UPDATE_FILENAME exists before applying update
- Add empty line at the end of files
Merge branch 'dev' into 411-update-system
Update system now uses HTTPS connection

- Add some curl checks
- Refactor code
Merge branch 'dev' into 411-update-system

Conflicts:
	constants.php
Check FRESHRSS_PATH is writable.

FRESHRSS_PATH needs to be writable before performing update.
Add data/last_update.txt

Remember last update timestamp.
CHANGELOG

Add new update system.

See https://github.com/marienfressinaud/FreshRSS/issues/411