Add base support for reading Entrezgene-set.

With this change, next_seq() will return a reference array where each
element is a sequence from an entrezgene set. This may cause problems
for people expecting to always get a single element on the array ref,
of for very large sets since they are all read into memory.
maint: update Mingyi Liu (original author) email address
doc: fix typo on documentation (rt.cpan bug #73965)