Refactor Scrolls

[asenchi]

This commit breaks backwards compatibility for Scrolls. It is a rework/refactor of the initial work done in the following PR: https://github.com/asenchi/scrolls/pull/54

• The #global_context is no longer mutable, instead we instantiate a class internally inside Scrolls::Logger that contains the global_context. This is to avoid various bad thread behaviors, that previous to Ruby 2.0, were allowed.
• The result of the above change removes #add_global_context.
• Currently Scrolls needs to be instantiated to be used (Scrolls.init(options)) however I'd like to work on a method for allowing one to just use Scrolls.log and get sane defaults.
• Due to this massive change I moved to "single line exceptions" by default since it lowers the amount of data produced by Scrolls when using #log_exception without losing the information.
• Officially kills Ruby 1.8.7 support (though literally no one should be using that version).

NOTE: I don't have a great setup right now to test everything here. I'm testing against Ruby 2.4.1 and all tests pass. Please test if you can. I'm going to cut a release of this version, 0.9.0.pre. I'm jumping versions here since we break backward compatibility. I'm hopeful with this and subsequent versions we can get to a place where a version 1.0 is a reality.

Please test this version as much as possible!

[asenchi]

This evening I worked to clean up a lot of the refactor, simplify some areas (especially around streams, facilities and structure).

Next I want to bring in any other PRs that existed, make sure they are well tested, appropriate and then work on updating all of the documentation. Then I think this is ready for a proper release.

[asenchi]

Alright, I've updated docs, cleaned up the code, added some tests and overall is am ready to merge this and release 0.9.0.