Add RPC call `getrawwallettransaction`

This allows getting raw transaction data from the wallet even if the transaction is no longer in the blockchain / mempool (for example if it got orphaned due to malleability abuse).

Allows a verbose flag just like `getrawtransaction`.

![getrawwallettransaction](https://f.cloud.github.com/assets/126646/2157991/eaf0f836-948a-11e3-9027-f155c980f07e.png)


Was also proposed in #3220, but is urgently useful now.