Prompt for cold wallet secret

Adds a dependency on promptly, but gatewayd no longer needs to have the cold
wallet secret on the command line (and thus in bash history or argv[0]).

Fixes #53
Merge pull request #303 from tdfischer/password-prompt

Prompt for cold wallet secret
Merge branch 'master' of github.com:ripple/gatewayd
[TASK] Extract capistrano to gatewayd/gatewayd-capistrano.
[TASK] Extract Docker functionaly to gatewayd/gatewayd-docker.
[FIX] set to a certain amount instead of always 0
[FIX] pass cold wallet address only to callback
[FEATURE] accept amount
[TASK] accept 0 if amount is not provided
[TASK] bump ripple-rest-client version
[FIX] Match correct routes to controllers methods
[TASK] Reorganize routes
[DOC] Fix typo and add information about superuser role in postgres
[FEATURE] DRY Resources CRUD router.

[TASK] Remove whitespace.

[FIX] Syntax error.
[FIX] Rename controller files and use correct resource routes
[TASK] Delete unused queues directory
[TASK] Remove unused behavior from lib/core.
Merge branch 'develop' of github.com:ripple/gatewayd into develop