[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
[FIX] Regression in image resize helper (Fixes #2529)

Commit 57ad514b makes the function preserve the aspect ration of the
original picture. Error of mine because the expected behavior was to
lose it for kanban view purpose.

For backward compatibility sake, this commit will keep the old behavior
by default.
[FIX] fields: inherited fields should get 'string' from their parent field

Because some parameters of a field may be determined during its setup, we have
to update the corresponding column after the setup, and recompute _all_columns
to make it consistent.