Add metapackage live-task-standard

Please consider this merge, to create a metapackage named live-task-standard. The motivation for the metapackage is to use it to create a 'standard' live image; see bug 876428.

This metapackage depends on packages with standard priority. The dependency list was initially created by mimicking tasksel's "standard task" package-selection logic, which is (roughly) this:

    grep-aptavail -s Package \( -F Priority standard \) --and --not \( -F Section '^lib|\/' \)

A few dependencies were manually pruned from the list, at the prompting of lintian.

The metapackage description was copied/adapted from tasksel's "standard task" description.