Save the export endpoint api version when parsing for use later
Add method for checking if export endpoint supports progress

This commit also stores the version in a more intelligent fashion.
Instead of just dumping it into a string, the version(of one is present)
is stored as an array [major, minor, micro]. Also, the title is now
stored in as an attribute in the export type checkbox labels.
Add export capability in both monitored and unmonitored mode.

The export mode used depends on the version returned from the export
endpoint on the server. If the version is new enough then we will
monitor downloads(check for a cookie) and be able to tell right away if
the download complete normally or in an error state. If there is not
support on the server for notifications in the response(notification =
cookie) then we call the downloads in a more serial manner and rely on a
simple timeout before making a one off judgement on the status of the
download. While the unmonitored approach certainly has some drawbacks,
it is the only way to make this backwards compatible with older versions
of Serrano that don't set a cookie in the response. The serial download
code is only used as a last resort when the export endpoint doesn't
support notifications on export completion.
Block export button while exports are in progress
Add an itunes-esque progress indicator for export downloads
Enable page range controls for user to export explicit page ranges
Update mininum Serrano version supporting notifications in response
Reset status to pending before starting export