Moving all the WebUI pkg download in fetch_packages.py including npm modules.

Conflicts:

	packages.xml
Adding format as 'file' which if specified then the file gets copied directly to dest
Updating SlickGridEnhancementPager.patch based on latest web-core.
Adding openSans.patch, to remove protocol field from URL to download http request while over https
Changes such that npm install will install the packages in tmp cache and from
there it will be copied to current directory/node_modules.
If already installed, it will not install again.
Adding patch update in SlickGridEnhancementPager.patch and adding some download files
Make fetch_packages work on mac
xen: Fix build break

python fetch_packages.py breaks in xen builder as xen doesn't have npm tool. This is not even
required in xen builder. Hence adding fix to catch OSError (thrown if it can't find npm) and
bailout gracefully.

Signed-off-by: Anirban Chakraborty <abchak@juniper.net>