Issue #14 : Configurable output

[jeremp]

I've started working on the #14 issue.

I introduced two PrintStreams initialized by default to System.out and System.err.
I replaced the references to System.out and System.err to the correct PrintStream.

I've seen that error output was used most of the time to display stuffs that, in my opinion, are not errors (request and response headers, response body, ... ).
I think we could change it to use the standard output.

I thing printUsage() and printConfigSample() should keep on using System.out

[ghost]

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[ldrozdz]

Closing due to CLA issues. Will reopen once those are resolved.

[linux-foundation-easycla]

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[ldrozdz]

Reopening as #15 to fix CLA issues.