Several Fixes and Improvements

This PR contains some fixes and some improvements.

* __Make private ssh key not required in config.yml:__ In some cases ssh keys are not needed. For example in our deploy config we passthrough the keys from other computers, so we don't use the same keys that are stored in our deployer machines.

* __Make use of symfony process component in exec method of base deployer__: In some parts of the code (CLISshProxy) it's used the symfony process component. In other parts is php exec that is used. The main idea is to try to standarize it. That's why I've added the process component in Base deployer's exec.

* __Add files helper to base helperset__: Files Helper was not loaded by default in base helper. It seems very useful to have it initialized. I think all symfony 2 deploys can make use of filereplacepattern function.

* __Add copy function to file helper__: I've added a copy function just to encapsulate things a little bit because a copy function is quite useful.

* __Changes in documentation__: Changes in documentation to reflect changes made in code.