Improved WindowsSecureMimeContext
Reduced code duplication
Only OpenPGP really supports sign+encrypt, so drop it from the S/MIME context
Modified the Decrypt() API to not produce signatures (this only makes sense for OpenPGP)
Fixed the build
Fixed parsing of empty multiparts

Fixes issue #13
Improved error message when a private key could not be found to decrypt the S/MIME content
Let the system locate the proper cert/key pair to decrypt the encrypted content
updated docs
Modified DefaultSecureMimeContext to use an addressbook certificate store as well
Updated DefaultSecureMimeContext to actually use the addressbook for certificate lookups
Added unit tests for X509CertificateStore
Improved and expanded the OpenPGP API and updated docs
Use the ChainValidityModel instead of the PkixValidityModel
Use the signing time as the date value when building a cert chain
If building the certificate chain fails, set the SecureMimeDigitalSignature.ChainException
Bumped version to 0.13.0.0
Fixed MimeKit.nuspec xml
According to rfc2045, the media subtype is a token, not an atom.

Fixes issue #14
Added more helpful code examples to the README