The /exports API now works with optional friendly name after /{ident_hash}

For example, the following URL will work:
/exports/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7.pdf/college-physics.pdf
Change extras API to return a download url with the friendly filename

For example, change from
/exports/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7.epub
to
/exports/e79ffde3-7fb4-4af3-9ec8-df648b391597@1.7.pdf/college-physics-1.7.pdf
Merge pull request #136 from karenc/export-friendly-name-url

Export friendly name url