Click-odoo

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- clarify the behaviour of ``env_option`` ``with_database`` and ``database_required``
parameters; in particular, when ``with_database`` and ``database_required``
are both set (the default), the ``--database`` option can be omitted
as long as a database is declared in the Odoo configuration file.

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- refactor the OdooEnvironment class: it is much cleaner when
it leaves the global Odoo config alone, so we completely move
responsibility to initialize the Odoo config to the CLI part.

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- better error logging and handling: all exceptions occuring
in scripts under click_odoo.env_options are logged and converted
to ClickException so we are sure they are both in the log file
and on the console (handled by click) for the user to see.
The OdooEnvironment context manager does not do additional logging,
leaving that responsibility to the caller.

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- close db connections when releasing OdooEnvironment
- expose click_odoo.odoo_bin (odoo or openerp-server depending on Odoo series).
not documented yet, because it should ideally be a full path corresponding
to the installed click_odoo.odoo, and I'm not sure how best to detect it yet.

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- minor documentation improvements
- add the possibility to run script without ``--database`` (ie without env,
but with a properly initialized Odoo library such as addons path)
- be more resilient in case we can't obtain a context for the user

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- click_odoo now exports the odoo namespace: ``from click_odoo import odoo``
is an alias for ``import odoo`` (>9) or ``import openerp as odoo`` (<=9)
- add a ``with_rollback`` option to the ``env_options`` decorator, to control
the presence of the rollback option
- document the ``env_options`` decorator