[FIX] account: fiscal position may map single tax to multiple taxes

(This is a regression in Odoo 8)

It should now work when a fiscal position replaces 1 tax with
several taxes (for example : Fiscal position "Intra-EU B2B" in l10n_fr)

Fixes #2261, manual merge of PR #3316
[FIX] Error in excel export when sheet name > 31 characters

XLWT only accepts sheet names smaller than 31 characters because that's the
excel limit.

Fixes #2548
Fixes #3318
[ADD] Stripe-style APIDoc: content
[ADD] doc: language switcher
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64