Add -v global flag for verbosity

Verbosity: 1 => print HTTP requests; 2 => print HTTP requests/responses

Fixes #959 

E.g.:

```
❯ tsuru -v 2 app-info -a someapp
*************************** <Request uri="/apps/someapp"> **********************************
GET /apps/someapp HTTP/1.1
Host: tsuru.corp.surveymonkey.com:8080
Connection: close
Authorization: bearer 6efcfbba38127d5f3c9a7121a8833e76ccb7f5cc

*************************** </Request uri="/apps/someapp"> **********************************
*************************** <Response uri="/apps/someapp"> **********************************
HTTP/1.1 404 Not Found
Connection: close
Transfer-Encoding: chunked
Content-Type: text/plain; charset=utf-8
Date: Sat, 27 Dec 2014 00:50:34 GMT
Supported-Crane: 0.6.0
Supported-Tsuru: 0.14.0
Supported-Tsuru-Admin: 0.8.1

17
App someapp not found.

0

*************************** </Response uri="/apps/someapp"> **********************************
Error: App someapp not found.
```
You should use the `Context.Stdout` to send the output. There is an example here: https://github.com/tsuru/tsuru/blob/master/cmd/cmd.go#L337