Add plugin to collect test results as JSON.

Run Avocado and return the test results as JSON.

Note: This PR is an improved version of collector.py inside the PR #62 with unit test included.

Example:

avocado --collect run 'sleeptest failtest'
<pre>
{
    "tests": [
        {
            "test": "sleeptest.1",
            "url": "sleeptest",
            "status": "PASS",
            "time": 1.200882911682129
        },
        {
            "test": "failtest.1",
            "url": "failtest",
            "status": "FAIL",
            "time": 0.19649291038513184
        }
    ],
    "errors": 0,
    "skip": 0,
    "time": 1.3973758220672607,
    "debuglog": "/home/rmoura/avocado/logs/run-2014-05-15-15.07.46/debug.log",
    "pass": 1,
    "failures": 1,
    "total": 2
}
</pre>
Signed-off-by: Ruda Moura <rmoura@redhat.com>