Follow meta-linter recommendations for all of Inbucket

- Rename BUILD_DATE to BUILDDATE in goxc
- Update travis config
- Follow linter recommendations for inbucket.go
- Follow linter recommendations for config package
- Follow linter recommendations for log package
- Follow linter recommendations for pop3d package
- Follow linter recommendations for smtpd package
- Follow linter recommendations for web package
- Fix Id -> ID in templates
- Add shebang to REST demo scripts
- Add or refine many comments

etc/mailbox-list.sh

@@ -1 +1,2 @@
+#!/bin/sh
 curl -i -H "Accept: application/json" --noproxy localhost "http://localhost:9000/mailbox/$1"

etc/mailbox-purge.sh

@@ -1 +1,2 @@
+#!/bin/sh
 curl -i -H "Accept: application/json" --noproxy localhost -X DELETE http://localhost:9000/mailbox/$1

etc/message-body.sh

@@ -1 +1,2 @@
+#!/bin/sh
 curl -i -H "Accept: application/json" --noproxy localhost http://localhost:9000/mailbox/$1/$2

etc/message-delete.sh

@@ -1 +1,2 @@
+#!/bin/sh
 curl -i -H "Accept: application/json" --noproxy localhost -X DELETE http://localhost:9000/mailbox/$1/$2

etc/message-source.sh

@@ -1 +1,2 @@
+#!/bin/sh
 curl -i -H "Accept: application/json" --noproxy localhost http://localhost:9000/mailbox/$1/$2/source