SSLv3 change to TLS

Apple dropped support for SSL 3.0 on push notification servers
Resolved ambiguous call to X509Certificate2 constructor

Casted X509Certificate2 constructor null password parameter to string to
resolve ambiguity between overloaded types (string / SecureString).
Removed useless references

Removed useless assembly references and using statements (Framework v2.0
can now be targeted)
Merge pull request #65 from TalAloni/master

TLS instead of SSL3 and compilation fixes