The User Interface's "Search" box accepts a space-separated list of predicates of the
form "[-]operator:value[:value]". These predicates are combined using conjunction.
That is to say we only have 'and' and atom negation for now.

Negation of a predicate is achieved by prepending a minus sign: -is:landscape will match
with pictures of not landscape ratio.

For example

    after:"2010-01-01" before:"2010-03-02T12:33:44" -loc:"Amsterdam" 

will return all images having "modtime" between before and after and not in Amsterdam.

When you need to match a value containing a space, you need to using double quotes around
the value only. For example: tag:"Three word tagname" and not "tag:Three word tagname".
If your value contains double quotes you can use backslash escaping.
For example: attr:bar:"He said: \"Hi\"" 

Usable operators:
    after:         date format is RFC3339, but can be shortened as required.
    before:        i.e. 2011-01-01 is Jan 1 of year 2011 and "2011" means the same.
    attr:          match on attribute. Use attr:foo:bar to match nodes having their foo
                   attribute set to bar.
    format:        file's format (or MIME-type) such as jpg, pdf, tiff.
    has:location   image has a location (GPSLatitude and GPSLongitude can be
                   retrieved from the image's EXIF tags).
    loc:           uses the EXIF GPS fields to match images having a location near
                   the specified location.  Locations are resolved using
                   maps.googleapis.com. For example: loc:"new york, new york" 
    is:image       object is an image
    is:landscape   the image has a landscape aspect
    is:pano        the image's aspect ratio is over 2 - panorama picture.
    is:portrait    the image has a portrait aspect.
    height:        use height:min-max to match images having a height of at least min 
                   and at most max. Use height:min- to specify only an underbound and
                   height:-max to specify only an upperbound. 
                   Exact matches should use height:640-640
    tag:           match on a tag
    width:         use width:min-max to match images having a width of at least min 
                   and at most max. Use width:min- to specify only an underbound and
                   width:-max to specify only an upperbound.
                   Exact matches should use width:480-480
    childrenof:    Find child permanodes of a parent permanode (or prefix
                   of a parent permanode): childrenof:sha1-527cf12