nested hstore patch

Hi!

Attatched patch adds nesting feature, types (string, boll and numeric values),
arrays and scalar to hstore type.

All new features are described in PGConf.EU talk
http://www.sai.msu.su/~megera/postgres/talks/hstore-dublin-2013.pdf (since PGCon
some features was added).

Patch includes:
1 implementaion SRF_RETURN_NEXT_NULL()
2 contrib/hstore changes
3 docs of new hstore module (many thanks to  David E. Wheeler
<david.wheeler@pgexperts.com>)

In current state patch is in WIP status, for short period I plan to move support
of binary nested structure to core to share binary representation for hstore and
json types.



--
Teodor Sigaev                                   E-mail: teodor@sigaev.ru
                                                    WWW: http://www.sigaev.ru/