Replace avhttp with libcurl: Some fixes:

1. Fixed an assumption somebody originally made in the plugin that std::string had contiguous storage. This is not specced behavior pre C++11 so we gamble by calling .reserve() which should give a far better guarantee.
2. Added copy to clipboard information for curl
3. Removed some openssl references in compiling.md
4. Renamed struct vars to be uppercase to match "public var" code style policy

pcbnew/github/github_plugin.cpp

@@ -538,7 +538,8 @@ void GITHUB_PLUGIN::remoteGetZip( const wxString& aRepoURL ) throw( IO_ERROR )
     try
     {
         kcurl.Perform();
-        m_zip_image.assign(kcurl.GetBuffer()->payload, kcurl.GetBuffer()->size);
+        m_zip_image.reserve( kcurl.GetBuffer()->Size );
+        m_zip_image.assign( kcurl.GetBuffer()->Payload, kcurl.GetBuffer()->Size );
     }
     catch( const IO_ERROR& ioe )
     {