API, DOC, MAINT: improve passive samplers

[stsievert]

This PR comes from more feedback from experimentalists.

What does this PR implement?

• API: modifies RoundRobin to be user-specific.
• API: allows specifying valid targets via targets for RoundRobin, Random and RoundRobinPerUser
• ENH (for the experimentalist): allows controlling queries more closely (e.g., "the first query shown to the user should {have indices [1, 2, 3], be from the Validation sampler}"

Other changes include the following:

• DOC: Adds note about asking "obvious" questions
• MAINT: shuffles round robin head order
• DOC: adds warning about key pair when launching on EC2

See commit message for more complete list of changes.

Reference issues/PRs Closes #128 Closes #130 Closes #131

[stsievert]

I think this release qualifies as a patch release, not a minor release. It's only HTML query page stuff (which isn't Salmon proper) and doesn't introduce any backwards incompatibility.*

^{*Unless hitting /query?ident=foo, which should have been fixed a while ago}