Move initializers executions into the proxy constructor logic

[facuspagnuolo]

Currently, when a new proxy is created by the UpgradeabilityProxyFactory it is first instantiated and then initialized. We can move the initializer method call within the constructor logic of the Proxy.

Note that we can do the same for the proxy upgrades.
This will allow us to deprecate the proxy factories.
This issue could be considered as part of zeppelinos/zos-lib#210