Allowing different archive format types

[violuke]

Allowing different archive format types. Backwards compatible as default is tar.gz which is what was the default before.
Correcting incorrect comment about "content" key in array too.

[violuke]

Note, I don't believe that the failed test here is related to my change:

There was 1 failure:

1) Gitlab\Tests\Api\ProjectsTest::shouldGetOwnedProjects
Expectation failed for method name is equal to <string:get> when invoked 1 time(s)
Parameter 0 for invocation Gitlab\Api\AbstractApi::get('projects?owned=1', Array (...), Array ()) does not match expected value.
Failed asserting that two strings are equal.

--- Expected
+++ Actual
@@ @@
-'projects/owned'
+'projects?owned=1'

/home/travis/build/m4tthumphrey/php-gitlab-api/lib/Gitlab/Api/Projects.php:56
/home/travis/build/m4tthumphrey/php-gitlab-api/test/Gitlab/Tests/Api/ProjectsTest.php:70