refactor: Add EventEmitter, remove callbacks, and bug fixes

[darinspivey]

BREAKING CHANGE: In general, callbacks do not work with a "flushing" app due to the asynchronous nature of HTTP requests and dealing with errors and retries. Using an EventEmitter is much more appropriate.

CHANGE LIST:

• Removed debug since it's not compatible everywhere. See issue (https://github.com/logdna/nodejs/issues/89)
• class Logger forces the use of the new keyword
• Helper functions are broken out to declutter the class file
• configs.js was renamed to the more appropriate constants.js
• Made all variables consistent in naming convention (lowerCamelCase)
• Added EventEmitter for successes, warnings and errors
• Fixed retry logic and possibility for double timers
• Fixed race condition with clearing a single buffer. Now, multiple buffers are used so that they can be independently cleared. This fixes a race condition where lines that were added during the HTTP request could be removed without being sent
• log() Options as a string must now be a log level. TypeError if not.
• Properly handles opts.meta, opts.context and this.meta according to indexMeta
• Removed sizeof since it was buggy. Logic replaced with .length of the lines
• Added meta to the constructor so that it can easily be set without needing to call addMetaProperty after instantiation.
• Fixed map transitions; No dynamic addition of object properties or delete usage.
• ip can now be an IPv6 address
• Added a loadtest.js test to ensure there is not data loss
• Exponential Backoff with Jitter algorithm implemented for HTTP retries

Semver: major Ref: LOG-6626

[darinspivey]

Closing this PR in favor of a new repository name and package namespace.