added RequestedAuthnContext to sp AuthnRequest https://issues.jboss.org/browse/PRODMGT-503

Example Configuration:
```xml
      <Handler	class="org.picketlink.identity.federation.web.handlers.saml2.SAML2AuthenticationHandler">
	     <Option Key="AUTHN_CONTEXT_CLASS_REF" Value="urn:oasis:names:tc:SAML:2.0:ac:classes:Password:JBoss,urn:oasis:names:tc:SAML:2.0:ac:classes:Password:JBoss2" />
             <Option Key="AUTHN_CONTEXT_COMPARISON" Value="exact" />
      </Handler>
```
Comma separated class refs and comparison value as described in the standard (“better” | “exact” | “maximum” | “minimum”). Comparison is optional also if you set AUTHN_CONTEXT_CLASS_REF.

Example authnRequest obtained from the configuration:
```xml
    <samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
                    xmlns="urn:oasis:names:tc:SAML:2.0:assertion"
                    AssertionConsumerServiceURL="http://jboss-eap6.corp.generalisvil.net:8180/wsweb/"
                    Destination="https://namx.corp.generalisvil.net/nidp/saml2/sso"
                    ID="ID_dcb979ba-1681-4b2b-b8cb-ae5427789922"
                    IssueInstant="2013-08-26T20:42:30.980Z"
                    ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST"
                    Version="2.0"
                    >
        <saml:Issuer xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">http://jboss-eap6.corp.generalisvil.net:8180/wsweb/</saml:Issuer>
        <dsig:Signature xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">
            <dsig:SignedInfo>
                <dsig:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#WithComments" />
                <dsig:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
                 <dsig:Reference URI="#ID_dcb979ba-1681-4b2b-b8cb-ae5427789922">
                <dsig:Transforms>
                    <dsig:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
                    <dsig:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                </dsig:Transforms>
                <dsig:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
                <dsig:DigestValue>QpZl7ctT2IN8ejiHtXGrSTdSQvU=</dsig:DigestValue>
            </dsig:Reference>
        </dsig:SignedInfo>
        <dsig:SignatureValue>XSGOA4VhMVryJuyooRIhyjMHYruryxCAjKGEwbUZ7d429xRq7W0u7NUHvTXm7TMFreas+1rW5fU0OIf8aUq9zltVN6mjGK52csRGvmtXOd1PL1phS2sSplXt2Xxlra5cyv6bjMkbWa04me+K80frAD7WJlco7bcB78dqFFlD1PZ79G8s/WUmp5K2nSav1g967aSZHhyIzVBy4IxhzdGqJv5VLWknZzH18pFTxa+wGiXjGg2CYtuKSnc2D+X6CW02xJ8OfSJeexpQkk8RcbD3LM8cArBqD7Nb1EpZ5aXs6PDkC/8Nw1IrZK2IYI+Yop3hsnMhlw4esJ7t9EEybnKeOA==</dsig:SignatureValue>
        <dsig:KeyInfo>
            <dsig:KeyValue>
                <dsig:RSAKeyValue>
                    <dsig:Modulus>1Z+gdQhFcwaqFCCokxx6+usSrdNFtj64bkMvjnhXVS+GqB6BCXvQWjQROYEV2Yb24UH2+NpyUyMMWKgx7tWPGr+omoOgY10Tqd7OsMyXkuiPkTDiwPrekM1cxYbEIkyK0rRXXek4w1U2ozha0gJhKLrAh9LnPBFezQWrKa8cYAYBV3hb4zWfJLzQsj01N+WBhP1tX0aJ8X+bIiE7IhqkhxYRu4z8H0n0WReHzp9BvzsOxIPYMu7XF92vsdvhnm1FdCP0famwANrJRp2FKi9pv/ktW1OZRl4227+W1ztVo83CliodLo3gSCZE19BzHGFYAJKX/nVd58g/uUFX+7bPow==</dsig:Modulus>
                    <dsig:Exponent>AQAB</dsig:Exponent>
                </dsig:RSAKeyValue>
            </dsig:KeyValue>
        </dsig:KeyInfo>
    </dsig:Signature>
    <samlp:RequestedAuthnContext Comparison="exact">
        <saml:AuthnContextClassRef xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">urn:oasis:names:tc:SAML:2.0:ac:classes:Password:JBoss</saml:AuthnContextClassRef>
        <saml:AuthnContextClassRef xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">urn:oasis:names:tc:SAML:2.0:ac:classes:Password:JBoss2</saml:AuthnContextClassRef>
    </samlp:RequestedAuthnContext>
</samlp:AuthnRequest>
```