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.
Add Image#signature attribute

This signature uniquely identifies an image, and can be use to determine
if an image has been modified, or compare two images.
Break #signature out into own method for perf

As calculating the signature requires reading the whole file, this won't
be a constant time operation. Therefore, be more lazy about it and only
calculate it when necessary.
Implement object equivalence for Image

Two MiniMagick::Image instances are equal (#== or #eql?) iff they are of
the same Class and their #signatures match. To be a well-behaved Ruby
object, also implement #hash, based on the #signature.
Merge pull request #270 from stevenharman/add_image_signature_attribute

Add Image#signature attribute
Allow EXIF data to have commas in the value

Except for a handful of fields known to encode their data in a
comma-separated list of character values -- ExifVersion and
FlashPixVersion. This allows other values, like GPSLongitude,
GPSLongitude, BitsPerSample, etc... to be extracted with incorrectly
"decoding" them. It fixes #36, where GPS info often caused EXIF data
extraction to break due to the numbers not being valid character codes.
Merge pull request #271 from stevenharman/allow_commas_in_general_exif_data

Allow EXIF data to have commas in the value
Add Rdoc example for MiniMagick::Tool#initialize
Bump to 4.0.0
Remove note about the RC in the readme
Remove test files from gemspec manifest

The images take up a lot of space, and there is no need to have test
files included in the gem.