Extended hosts and host metrics api methods

[yoheimuta]

I want to call /api/v0/hosts.json with any query parameters and /api/v0/hosts/{host_id}/metrics.

Usage examples:

my @args = (service => "service", role => "bastion", 
    role => "batch", status => "working");
my $content = $mackerel->get_hosts(\@args);

my $host_id = "2RnaLGypmBJ"; 
my $args = { name => 'loadavg5', from => '1478744400', to => '1479303590' }; 
my $content = $mackerel->get_host_metrics($host_id, $args);

[yoheimuta]

sorry, I'm going to fix tests.

[yoheimuta]

sorry, I'm going to fix tests.

It's an unrelated issue #6 with this pr, so I reopen it.

[cou929]

@yoheimuta The diff looks good but CI has been failed as you mentioned.
I commented to #6 about that.

[yoheimuta]

@cou929 Thank you for advice. 135ee56 fixes a CI failure.

[cou929]

@yoheimuta I reviewed 135ee56 and It's okay.
I think you can merge this p-r.
Thank you!

[yoheimuta]

Thanks !