creation and implementation of data store

not fonctionnal
Merge branch 'master' into data-store

Conflicts:
includes/functions.php
public/class-linkedin-oauth2.php
made datastore singleton

- SessionDataStore is now singleton
- added methods __toString in LinkedInUser and LinkedInToken
- rename LinkedInDaraStore:: getUser renamme
differents changes

- convert abstract LinkedInDataStore to LinkedInDataStore interface
- added abstract class DataStoreSingleton
- changed signature of SessionDataStore class
- clear logs
- bug fix get_linkedin_oauth_data()
test HTTPS key existence before testing its value
renamed LinkedInUser -> LinkedInData

- rename class
- update import
- renamed setData -> parseResponse
- data is now extra data
comments
removed unused commented function __sleep
fixed infinite loop call

- SessionDataStore::getToken() throws an exception if no data
- LinkedIn exception default parameters are optional
- get_linkedin_token() returns null if LinkedInTokenException is thrown
- added more control in is_linkedin_token_valid() and
is_linkedin_user_connected()
- added LinkedInTokenException class
- update plugin version
update plugin version number
change string to constant
fix the to get token data to build API rest query string
fix the to get token data to build API rest query string
removed unused class
Merge branch 'data-store'

Conflicts:
	includes/api/rest/class-linkedin-rest.php
	includes/functions.php
	public/class-linkedin-oauth2.php
	public/includes/functions.php