Applying updates mentioned in ticket #617

The documentation should be RFC2606, RFC3849 and RFC5737 compliant. And added documentation regarding the gmysql-password= option in pdns.conf.
Include a warning in the supermaster section

Some users coming to #powerdns ask questions and don't see the SOA isn't available on the slave as long as they don't run pdnssec rectify after a change.
Merge pull request #869 from nlmark/patch-2

Include a warning in the supermaster section
Merge pull request #867 from nlmark/patch-1

Applying updates mentioned in ticket #617
sql constraint clarification, thanks Markus Petzsch
Fix bug on getTSIGKey where name did not get bound, also remove
requirement name to match zone name.
Fixed bug where HMAC-MD5 is always assumed and fixed for.
Fix wrong order of bind and string_to_cbuf
Fix typo on line 604