two new matchers "be_mounted" and "be_resolvable"

hello :)

ive been playing around with serverspec today. i really like the idea of test servers with rspec. this is the first github project i contribute to ever and my first ruby project too. ive created two new matchers:

* be_mounted
* be_resolvable

usage example:

```ruby
describe 'resolv' do
  context "hostA" do
    it { should be_resolvable.by("dns") }
  end
  context "hostB" do
    it { should be_resolvable.by("hosts") }
  end
  context "hostC" do
    # use dns by default
    it { should be_resolvable }
  end
end

describe "/usr/local" do
  it { should be_mounted }
end
```
i hope i have done everything right. thx :smile: