Show the optional usage of combine_options in ::new

This feature was added for 4.0.0, but is not documented. Now it is!
Add respond_to_missing? to complement method_missing

The `#respond_to_missing?` hook is the complement of `#method_missing`,
used by `#respond_to?`, `#method`, etc... to do the right thing. In this
case, since `#method_missing` is implemented in terms of an instance of
`MiniMagick::Tool::Mogrify`, we use an instance of that class to
determine if objects of this class can respond to those missing
messages.
Merge pull request #269 from stevenharman/add_responds_to_missing_to_image

Add respond_to_missing? to complement method_missing
Merge pull request #268 from stevenharman/add_example_of_combine_option_block_for_new

Show the optional usage of combine_options in ::new
Put a small enough number for testing timeout

The Travis tests were sometimes failing, because a timeout error wasn't
raised, because the command executed too soon.
Remove the timeout test

I can't get it to work consistently on Travis, so it isn't worth it.