Utilize HTTP proxy for every HTTP request within mail auto-config + possibility to disable guessing.

Changed property names to better reflect generic purpose in 'autoconfig.properties':
com.openexchange.mail.autoconfig.ispdb.proxy -> com.openexchange.mail.autoconfig.http.proxy
com.openexchange.mail.autoconfig.ispdb.proxy.login -> com.openexchange.mail.autoconfig.http.proxy.login
com.openexchange.mail.autoconfig.ispdb.proxy.passw ord -> com.openexchange.mail.autoconfig.http.proxy.passwo rd

Added property com.openexchange.mail.autoconfig.allowGuess to 'autoconfig.properties' file to disable guessing of mail/transport settings. Default is true.