Add web service for exporting FASTA for region

[heralden]

It should give the same output as the [FASTA...] buttons in the JSP webapp region search results page.

Currently we only have web services to get the FASTA sequence for features overlapping a region, but there's no way to get the FASTA sequence for an arbitrary region.

PS: It would be nice if this web service also accepts multiple regions, like the top [FASTA...] button used to export FASTA sequence for all the regions in the search.