Support Postgres money column type

[plicjo]

Opened as a result of the discussion in this PR: #44 (comment)

With numeric types things are more complicated, since Postgres has the Money type, for instance, and it looks like it won't be detected as numeric. Then again, the old code might have been wrong too, since it detected Money as numeric, but not as decimal.
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With the current version of Rails it seems that the only missing type is :money from PostgreSQL which should be mapped to Decimal.

We need to add validation support for the :money column type in PostgreSQL as if it was a decimal column.

[boazy]

Fixed with 83ba3ab

[boazy]

I want to write unit tests for that, so I'm reopening a new issue for unit tests, so I've opened a new issue #46.