Allow passing locale to query methods

[shioyama]

Replaces #210, much simpler now.

[shioyama]

Here's what this does:

Post.i18n.where(title: "foo", locale: :ja)
#=> returns post with Japanese title "foo"

The interface here may be slightly confusing, since you pass the locale as an option to i18n when setting the locale for block queries:

Post.i18n(locale: :ja) { title.eq("foo") }
#=> returns post with Japanese title "foo"

but this locale keyword to i18n will be ignored when used with query methods:

Post.i18n(locale:ja).where(title: "foo")
#=> ignores locale and returns post with title "foo" in current locale

For the above case to work, we would have to store the locale in the chained relation, which I don't think I really want to do.