Add table for invoice extrafields
Prepare invoice admin page to add extrafield support
Add admin screen for extrafields on customer invoices
Add extrafields support when create a customer invoice
Edit extrafields on customer invoice
Retreive extrafield when fetch invoice
Show extrafields for object into invoice ODT document
Merge branch 'extra_invoice' into extrafields

Conflicts:
	htdocs/install/mysql/migration/3.3.0-3.4.0.sql
Fix : select correct value of extrafield (select list)
Add value to store price type extrafield without currency in ODT
Extrafields on ODT (propal) : fix value of price with currency & display date with correct format
New function to fill ODT substitution array with extrafield of object
Use price2num() function instead of price() to use calcul in ODT.
Libre/Open Office wants number with dot and no space to allow make calcul in ODT document