Add extensions support for webservice/webaccount

This is another patch in a series of patches for interacting with
online accounts from Sugar. The relevant feature request is [1].

This patch introduces two new sections in sugar extensions:

(1) webservice is stub for installing webservices; each web service is
installed in a subdirectory of this directory.

(2) cpsection/webaccount is a new control panel section for managing
web service accounts. Account management services are loaded into
individual subdirectories of cpsection/webaccount/services.

Note that this same directory structure can be set up in
~/.sugar/extensions for web services installed by the end
user. pkgutil is used to resolve namespace issues.

[1] http://wiki.sugarlabs.org/go/Features/Web_services

A unit test for the new cpsection will be included in a separate patch.