Re-work of the logging / debug bar panel feature.

- Main class pulled into a separate file, leaving main plugin file lighter (could still do with some work).
- Debugging (or more strictly, logging) features abstracted from main class into its own class. The main class still uses a handy debug_log() method, but this is just a wrapper for a separate logger class. An instance of the logger class is injected via a set_logger() method in the main class. This leaves the main class as just being the plugin functionality, with optional logging. This class has no knowledge of what is done with the logs (i.e. used in debug bar panel class).
- The logger class takes it's structure from PSR-3, with the intention that another PSR-3 compatible logger class could be used in the future with minimal changes. Note that the logger class has no interaction with WP function calls etc. so this class could be developed into a more generic Blazer_Six_Logger class (that covers all log level methods used in PSR-3) for use in other projects or WP plugins. This class has no dependencies and no knowledge of how the logs were collected or what will be done with them.
- The Debug Bar panel class only deals with the logger class, and is somewhat dependent on the structure under which the logger class stores each log. It has no knowledge of how the logs were collected, or even what they apply to. This could be made into a more generic Blazer_Six_Debug_Bar_Panel class, and then extended to make it specific for this plugin (e.g. panel title, styling).

Other changes:
- Main class no longer uses a singleton pattern. It is instantiated within the main plugin file.
- wp_embed_handler() method adds an oembed="1" attribute, so that grouping of log messages can take account of whether a raw oembedable URL was used, or the direct shortcode that the URL would have converted to.
- Debug bar panel has amended styling - keeps each message as a string (as per PSR-3), groups messages by a hash of the processed shortcode attributes, includes a respresentation of the raw shortcode as group title.
- Debug bar panel hidden when there are no log messages to show.
- Fixes https://github.com/bradyvercher/wp-blazer-six-gist-oembed/issues/3 for correct line highlighting, and improves it by adding trim()s, so that values like " 1, 5,9 -13" will work like "1,5,9-13".
- Similar trim()s added for line number attributes.
- Enqueuing of style sheet changed to correct hook.
- Added localization support (no .po/.mo generated yet).
- Documentation improvements.
- Addition of me as an author, due to significant code changes.
- Version bump to 1.2.0.
Revert version bump to 1.2.0, as 1.1.0 has not been released yet.
Refactor main plugin file.

The instantiation of main and logging class is kept in the global scope, but the injection of the logging class into the main class, and the calling of the main class run() method that hooks in all of the other actions and filters, is now done within a function hooked to the init action.

The loading of the plugin text domain / localization is also improved, and hooked into the init action as well (but in a different function).
Add if / foreach optional braces in, to match existing coding style (with one eye on PSR-2).
Minor formatting and style tweaks.