Allow to provide SecurityContextRepository and SecurityContextHolderStrategy to be used

[dodgex]

In this PR I added fields for a SecurityContextRepository and a SecurityContextHolderStrategy und use them similar to AbstractAuthenticationProcessingFilter#successfulAuthentication to store the SecurityContext in the SecurityContextRepository. By default it uses RequestAttributeSecurityContextRepository wich is the same as in the AbstractAuthenticationProcessingFilter.

To finally achieve the expected result for #185 we have to configure the SpnegoAuthenticationProcessingFilter with at least a HttpSessionSecurityContextRepository or better (as far as I can tell from checking what spring-security is doing) with new DelegatingSecurityContextRepository(new RequestAttributeSecurityContextRepository(), new HttpSessionSecurityContextRepository()). Using http.getConfigurer(SecurityContextConfigurer.class); should also work if the http object is available when creating the SPNEGO Filter.

Fixes #185

I have signed and agree to the terms of the SpringSource Individual Contributor License Agreement.