app-pouchdb-query selector improvements

[ahodgkinson]

This pull request implements improved processing of the app-pouchdb-query selector attribute:

• JSON parsing of selector strings
• Use of Javascript objects a selectors

The existing parsing functionality is unchanged.

Note that the current parsing does not allow the full fidelity of pouchdb-find to be expressed as a selector string. In particular, embedded strings with spaces in them cause invalid selector objects to be created.

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[ahodgkinson]

Added a CLA for my alternate email address. This should now be good to go.

[googlebot]

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