added column for storing last login timestamp
code modified for last_login

#Modifications:
1. When new user created entry for last login inserted
2. Last_login updated when existing user object is used
3. public function to retrieve last_login timestamp for a user
function comment corrected!
Merge branch 'last_login' of github.com:mebjas/phpsec into mebjas-last_login

Conflicts:
	libs/auth/user.php
Changes function names to `get*Timestamp` from `get*Date`
Merge branch 'mebjas-last_login'