[FEATURE] In Send/Convert, user should be able to specify the gateway (RT-1790)

**WARNING**: XAU/demmurage currencies will cause fatal error due to a bug in ripple.js/currency.js. So just hold on  this PR before ripple/ripple-lib#127 merges.

![image](https://cloud.githubusercontent.com/assets/158528/3868106/385b24c4-2023-11e4-8b1b-565f211e3f98.png)

[RT-1790](https://ripplelabs.atlassian.net/browse/RT-1790)
[BountySource](https://www.bountysource.com/issues/2842505-during-the-send-convert-flow-user-should-be-able-to-specify-the-gateway)