conditionalize to avoid deprecation in 2.0.0
Merge pull request #51 from tenderlove/2.0.0

conditionalize to avoid deprecation in 2.0.0
Add note about needing libffi.
Merge pull request #53 from aripollak/patch-1

Add note about needing libffi
Don't depend on active support

see: https://github.com/jonleighton/spring/commit/42b03dbfae60fc333a5a5c3bdbf7dc3b5b487d3c#commitcomment-2607109
fixes #52
Merge pull request #54 from amatsuda/homemade_class_attribute

Don't depend on active support
Reimplement preloads again

* This version is a bit simpler.
* Changed the test so it doesn't alter global scope.
* Made sure that << doesn't mutate the superclass' preloads.
* We don't need to worry about what happens when
  Spring::Commands::Command.preloads is modified - people can add code
  in any number of places (e.g. application.rb) if they want it to run
  globally
Remove hard dependency on fiddle

On Ruby 2 we can use Process.getsid (thanks @fumiyas for implementing
it!)

If fiddle is not present, we can shell out which is a tad slower, but
better than requiring people to install dependencies and rebuild their
ruby.

Thanks @rjackson for researching these alternatives.

Closes #20.
Fix tests to make them run out of the box on different rubies
Fix race condition
Test against Ruby 2
Add dynamically generated help output.