Support HTTP authentication when specifying .rosinstall file

[mikepurvis]

Suggest some combination of the following:

• Interactively prompt for user/pass as necessary.
• Accept --username, prompt for password.
• Accept http://user:pass@server/foo.rosinstall format.

Would be useful for internal development, unreleased software, etc.

[dirk-thomas]

Closing since the repository is about to be archived: see #126.