Feature/add xtrans cols

## Purpose
Update db migrations for new cols, make deprecated cols not required, small bug fixes

#### New
- memos
- source_amount
- source_currency
- source_account_id (was from_account_id)
- destination_amount
- destination_currency
- destination_account_id (was to_account_id)

#### Deprecated, not removed
- amount
- currency
- deposit
- external_account_id

Please add validation for invoice_id to be SHA-256 hash. Implemented here: http://goo.gl/r7bga3