Make it possible to extends classes responsible for HttpClient creation

In order to customize the creation of the HttpClient
for needs such as configuring proxy and additional things from the Properties file
and even use Apache HttpComponents 4.3

Needs to change HttpClientFactoryBean and StdHttpClient 
to avoid using private fields

In order to minimize the code change, I used protected modifiers instead of private.
public getters could have been used instead but would have resulted in a bigger changeset.