Do not include SSHKit::DSL in the context it is required in

[beatrichartz]

In #28 the dsl file was removed from the original requires - I assume since the DSL was also automatically included in the context it was required in.

If I want to load the DSL and use it in another context than required in, the current inclusion pattern for the DSL leaves no options - the DSL gets auto-included wherever I require it.

This leads to code like:

require 'sskit'
module MyModule
  class ClassUsingDSL
    require 'sshkit/dsl'
    #...
  end
end

which is suboptimal. We can require the dsl with all other files and make the include optional instead, which allows for:

require 'sshkit'
module MyModule
  class ClassUsingDSL
    include SSHKit::DSL
    #...
  end
end

The required changes are in this PR.

[leehambley]

@mattbrictson any reasons you see not to merge this? I missed it, but it seems like a win.

[mattbrictson]

@leehambley This looks good to me. I was worried about this breaking Capistrano, but it looks like Capistrano never actually uses the SSHKit::DSL; instead it defines its own on method that does something similar. So I think that means we are safe.

We need a CHANGELOG and a rebase for this PR.

[leehambley]

@beatrichartz could you take care of that please?

[beatrichartz]

@leehambley done