svn export use --ignore-externals option if specified :externals => false

Some svn repositories have `svn:externals` that refs to `http://` URL.
`svn export` fails when I'm behind a http proxy that blocks HTTP `PROPFIND`, and that cause a failure at `pod install`. (such proxy is on my company network... :worried:)

With `--ignore-externals` option, `svn export` and `pod install` succeed and I can separate svn:externals into pod's dependencies.

So I'd like to suggest `:externals` option like this:
`s.source   = { :svn => 'https://google-api-objectivec-client.googlecode.com/svn/trunk', :revision => 'r301', :externals => false }`

For backward compatibility, svn downloader updates externals without `:externals => false` option.

Similar option for git submodules was merged here:
CocoaPods/CocoaPods#442