Generate invoice as pdf and customize filename

[jbgury]

Purpose of this improvement :

• allow to customize the filename of the file when saving invoice
• generate the invoice as pdf file

Keep the possibility to print, but add the save as pdf option.

Be carefull with the template of file, to not create security hole with traversal directory.

Limit the fields to compose the file name with :

• name and firstname of the patient
• invoice date
• invoice number Character to be included in the name: "_-"