Composed look

The main change was to extend the SetComposedLook() and SetCustom/MasterPageUrl() functions so they are applied recursively to subwebs where the inherit flag is set, similar to the corresponding operations in the web UI, and with an option to reset inheritance.

Note that publishing sites are set to inherit by default (other sites are not).

The names of the functions were also changed, as the term "theme" is specific to the .spcolor palette file (and the theme catalog it is stored in). The web UI uses the term "composed look" and the relevant library is called the "design catalog".

Unit tests were added for the new functionality and existing tests were simplified a little (with some bug fixes/issues applied).

An issue with the DesignPackage.Install() function, meant for installing sandbox solutions, was discovered that it wipes out the design catalog items when used. This appears a bug in the client framework code. 

The corresponding tests have been set to Ignore(), otherwise it breaks other tests (that are relying on the OOTB composed looks to exist).