Support mapping `xsi:nil` marked elements as `null`s (`JsonToken.VALUE_NULL`)

[cowtowncoder]

Although XML specification does not specify semantics for xsi:nil (with appropriate namespace), XML Schema specification does, and so some Java XML libraries (including JAXB) support mapping it into nulls.
(there is the related question of generation/writing, but this issue covers only read side).

It would be good if Jackson did same mapping automatically.

For background, see

• Handling XMLElement nillable like JAXB #89 (nil like JAXB)
• Exception on deserializing empty element with an xsi attribute on non String type. #184 (for BigDecimal etc values)
• Only self-closing tag with 'xsi:nil="true"' can be mapped #293 (nil for Collections)
• Xsi nil in collection fails to deserialize #353 (nil in Collections)

[cowtowncoder]

Note: one follow-up task, #359 -- but closed for 2.x