Added XID to logout URL

The link generated by {exp:freemember:logout_url} in EE 2.7+ fails because it does not include an XID. An XID is usually only required for POST submissions. However, since the act_logout method utilizes member_logout from the core EE member class, the secure_forms_check filter applies in this GET context. Not wanting to hack the core, I've added the XID hash to the logout URL that logout_url generates to bypass this issue.
Merge pull request #48 from mark-cr/logout_url_fix_ee27

Added XID to logout URL
Update register_tag.md
Add XID to logout URL to support secure forms in EE 2.7
Only add XID if secure forms is enabled
Fix returning to /index when user logs out from homepage
Update CHANGELOG.md
Version bump
Update README.md
Merge branch 'master' of https://github.com/expressodev/freemember into develop

Conflicts:
	system/expressionengine/third_party/freemember/mod.freemember.php
Re-adding tweaked Remember Me feature