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

[banjun]

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... 😟)

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

[alloy]

Very nice! Can you first add an entry to the CHANGELOG crediting yourself?

[coveralls]

Coverage remained the same when pulling 6ddf47e on banjun:svn-ignore-externals into 4a95877 on CocoaPods:master.

[alloy]

Hmm, it seems the Travis CI build has been failing since a recent change of mine, please ignore that.

[alloy]

Thanks!