Fixed set_cookie max-age parsing

Max-age is expected to be in seconds and thus cannot be parsed by _since_epoch.
Expire attribute is calculated from now() and max-age
Added allow_unsolicited parameter to config class.
Merge pull request #134 from Runscope/master

Fixing location of 'allow_unknown_attributes' in args lists
As Rainer Hörbe pointed out: "A principal is a user, not an entity in SAML. The UnknownPrincipal exception in mdstore.py should be renamed as suggested in the title to avoid confusion.
Unfortunately the OASIS SAML glossary is using the X-811 definition of principal, but contradicts itself otherwise by redefining the term implicitly in other definitions such as Account, Affiliation, IDP, Persistent Pseudonym, etc"
Merge branch 'master' of github.com:rohe/pysaml2
Update request.py
Merge pull request #136 from mdipierro/master

Update request.py. Take care of the possibility no receiver_addrs are defined.
Merge pull request #123 from tpazderka/cookie_expire_fix

Fixed set_cookie max-age parsing
Endpoint index when given must be an integer but stored as a character.
Editorial
Allow kwargs to percolate down through calls. Mark static methods as such.
Corrected documentation.
Merge branch 'master' of github.com:rohe/pysaml2