Cope with duplicate name->IP entries

The zone DB no longer considers duplciates of (IP, name); it will
reject duplicates of (IP, name, ident), but this ultimately doesn't
matter since those are all the fields it records anyway.

The HTTP interface treats these duplicates as trivial success (i.e.,
it acts as if it's idempotent, which it is).

This changes the behaviour for removals: if (IP, name) is recorded for
two different containers, removing one will leave the other extant.
fix typo
Merge pull request #257 from squaremo/issue249_duplicate_dns

LGTM.  Closes #249
Add play button over screencast image, so it invites to click
Merge pull request #261 from errordeveloper/master

Add play button over screencast image, so it invites to click
Smoke tests

    cd test
    cat README.md
    ./setup.sh
    ./run_all.sh
Fixed typo in error message
Merge PR #241 from 'squaremo/smoke_tests'
Rename docs/testing.txt to avoid confusion now we have smoke tests
Explain multiple containers with same name

Two further scenarios described
Add `-tags net