Improved access to Current Employer value for contacts

Improved access to Current Employer value for contacts (this is defined as an Individual contact's newest active Employer relationship):

Add "employer_id" to civicrm_contact table. (Also update upgrade script)

So whenever we add current employer, we should store organization id to employer_id

Modify all exiting code to use this changes.

1. Include Current Employer (organization name) in default contact export set (PRIMARY fields).

2. Current Employer is available when "selecting fields" for export.

3. Current Employer is displayed (when populated) on Contact Summary screen (contact/view) in front of Job Title display.

4. Current Employer TOKEN is available to be included in Mailing Labels -

{contact.current_employer}