OPML import and export

[devongovett]

There should be an API to import and export an OPML file containing feeds and folders. Google reader uses the following APIs for this:

/reader/subscriptions/export
/reader/subscriptions/import

The export one is documented here and the import one is briefly mentioned here, though untested.

[devongovett]

Both importing and exporting are now working, partially thanks to @nto. We need to more thoroughly test both, but especially the importing with real OPML files, and then I'll close this issue.

[devongovett]

Alright, tests added. Closing.