Support for self signed certificates / custom TrustManagers

This is my proposition to fix #20. This also changes the effect of FLAGS_SSL_IGNORE_INVALID_CERT a bit, as it overrides previously custom installed TrustManager. Also, with this flag set each time verifyConnection() is called a new all-accepting TrustManager is created. Don't know if that's the best solution, but you decide. ;)

As the commit message already sais:
* renamed trustAllManagers to TrustManagers to reflect that
   this isn't necessary an all-accepting TrustManager anymore
* added installCustomTrustManager() to XMLRPCClient
* with FLAGS_SSL_IGNORE_INVALID_CERT set a custom TrustManager
   is overridden by the all-accepting version.