When a module uses `Trans`, two underscore functions will be added to provide
the necessary metadata for the `Trans.QueryBuilder` and `Trans.Translator`
functions.

This underscore functions are:
- `__trans__(:container)`, which returns the name of the translation container field.
- `__trans__(:fields)`, which returns a list of the translatable fields.

The translation container customization allowed users to declare an
alternative field (the default was `:translations`) as the field that
contained the translations for the schema.

This customization has been sacrificed to ease the rewrite of the
QueryBuilder module. Now, Trans will always look in the `:translations`
field and there is no way to customize it.

All the functions of the `Trans.QueryBuilder` module have been removed
and replaced with the macro `translated/2`. This macro generates
an SQL Fragment and can be directly used from inside an `Ecto.Query`.

This removes the necessity of building the query before calling the
`Trans.QueryBuilder` functions and integrates seamelessly with
`Ecto.Query` provided functions for comparisons, etc.
This also permits adding conditions on joined schemas.

The QueryBuilder tests have been updated to use the newly introduced
`translated` macro.

The translator module does not require to be passed the name of the
translation container field. The test have also been updated to reflect
this change.

In PostgreSQL 9.4 [1] the operators had different precedence rules than in
PostgreSQL 9.5 or higher [2] (take a look at "any other operator" in the table.

This differences caused errors when triggering a query that contained something
similar to `where a->b is not null`, which 9.4 would interpret as
`where a->(b is not null)` and 9.5 or higher would interpret as
`where (a->b) is not null`.

This commits adds parentheses around the JSONB accesses to avoid this kind of
problems by stating the desired preference explicitly.

[1] https://www.postgresql.org/docs/9.4/static/sql-syntax-lexical.html
[2] https://www.postgresql.org/docs/9.5/static/sql-syntax-lexical.html

Options provided when using `Trans` are now stored into a module
attribute.

`Trans` will validate the fields declared as translatable and raise an
error during compilation time if they are not declared in the module's
struct or schema definition.

Trans.Translator now validates that whether the field is translatable and
provides a useful error message instead of the default `KeyError`.

Docs have been updated and typespecs added.

The function `Trans.translatable?/2` checks whether a field is
translatable or not.

The macro `Trans.QueryBuilder.translated/3` now validates the translated
fields before the macro expansion step. We will receive an error in
compilation time when adding conditions on untranslatable fields.