change static_path_find to allow you to specify the amount to send.

[jedmccaleb]

remove the source_currencies field
add a field source_amount
if source amount has a value then the value can't also be set in the destination_amount field.

Now you can say I want the receiver to get 200 EUR. What amount of currencies do I need to send to him to get there.

This change allows you to say I have 1000 USD and I want the receiver to get EURs what is the path.