replace callbacks with procedural method

callbacks were being used to register where any plugins would execute
with relation to the main capture method, this replaces callback event
registration with a simple ordered list in a method.

closer to removing active support dependency, but this is actually
easier to read and understand IMHO (no more list of callbacks that
happen in reverse order)

also put some comments in to indicate areas where we don’t have any
plugins yet, but where they would go (should help with abstraction for
when we break out third party vs native plugins in future)
determine plugin run order based on plugin itself

Instead of explicitly defining each plugin that runs in the running,
each plugin that wants to be run by the runner should define
self.runner_order with either :precapture or :postcapture.

With this, we can also get rid of some of the ActiveSupport inflection
metaprogramming stuff that automatically creates method names for
runner, getting us closer to deprecating the active support gem
entirely.

NOTE: tests passing but haven’t manually verified with all the
different plugins yet.
use Object.const_get instead of constantize

Removes dependency on active_suppor/inflector
remove active_support/concern

This doesnt appear to actually be being used anywhere!
remove active support from gemspec

all usage of it seems to have been confined to what I just refactored
out of runner.

TODO: tests are passing but still need to audit the existing plugins,
as some of them may have taken advantage of the fact we had it loaded,
and since the plugins general have poor test coverage its possible this
breaks one.
move all capturers into a subdirectory

this has nothing to do with the rest of this PR but it was bothering
me so I am doing it here anyhow to clean things up.
chain const_get to handle module namespacing

const_get only handles automatically splitting on “::” to deal with
module namespacing on Ruby 2.0+, making the changes introduced in
d55ff95d fail on earlier versions.

Thus use a chained .const_get to handle getting the module first , then
the class within the module.
make separate precapture and postcapture execute phases

I feel like I’m missing something here and this could be a lot DRYer.
delete notes file i apparently left for myself two years ago
better error handling around error-prone plugin code
Merge pull request #204 from mroth/inactivesupport

refactor out activesupport dependency
adding Ruby 2.1.2 to travis config
Correct an error message
Merge pull request #210 from almirfilho/master

Correct an error message
--last includes animated gifs, fixes issue #213
Merge pull request #214 from mroth/fix-last-option-to-includes-gifs

--last includes animated gifs, fixes issue #213
remove @ mentions in commit messages
fix build
properly escape @s
Merge pull request #217 from csabapalfi/ignore_mentions

Escape @ characters in commit messages as they break imagemagick