Adding logs by stack

[bquenin]

Close #232

Verification

$ sudo ./swarm start
$ make
$ make test
$ ./amplifier

In another terminal:

$ ./amp stack up stack -f api/rpc/stack/test_samples/sample-04.yml
$ ./amp logs --stack stack

[freignat91]

Tested ok

[subfuzion]

@bertrand-quenin @freignat91 Some log tests are failing:

--- FAIL: TestShouldFilterByService (0.04s)
        Error Trace:    logs_test.go:120
        Error:          Should NOT be empty, but was []
        Messages:       We should have at least one entry

--- FAIL: TestShouldFilterByMessage (0.01s)
        Error Trace:    logs_test.go:131
        Error:          Should NOT be empty, but was []
        Messages:       We should have at least one entry

--- FAIL: TestShouldFetchGivenNumberOfEntries (0.22s)
        Error Trace:    logs_test.go:154
        Error:          Not equal: 111 (expected)
                                != 110 (actual)

        Error Trace:    logs_test.go:154
        Error:          Not equal: 121 (expected)
                                != 110 (actual)

        Error Trace:    logs_test.go:154
        Error:          Not equal: 131 (expected)
                                != 110 (actual)

        Error Trace:    logs_test.go:154
        Error:          Not equal: 141 (expected)
                                != 110 (actual)

        Error Trace:    logs_test.go:154
        Error:          Not equal: 151 (expected)
                                != 111 (actual)

        Error Trace:    logs_test.go:154
        Error:          Not equal: 161 (expected)
                                != 111 (actual)

        Error Trace:    logs_test.go:154
        Error:          Not equal: 171 (expected)
                                != 111 (actual)

        Error Trace:    logs_test.go:154
        Error:          Not equal: 181 (expected)
                                != 111 (actual)

        Error Trace:    logs_test.go:154
        Error:          Not equal: 191 (expected)
                                != 111 (actual)

FAIL
FAIL    github.com/appcelerator/amp/api/rpc/logs        11.540s
make: *** [test] Error 1