Convert codebase to ES6 and reduce dependency on Node-specific modules

[valadaptive]

This PR does a few things:

• Switches to ESLint for the linter. ESLint is a lot more flexible than JSHint, and adds several rules that let me more easily migrate away from ES5.
• Actually migrates the codebase to ES6. There is one breaking change here AFAIK:
  • API consumers which inherit from avro.Type classes (especially LogicalType) must use ES6 extends.
• Rearranges the code to be more conducive to running in the browser.
  • The dependency on util has been almost entirely removed. deprecate and debuglog are the only remaining use cases, and they're put behind shims that are no-ops in the browser.
  • The dependency on path has been abstracted behind the existing files.js compatibility layer.
  • The dependency on zlib has been removed by removing 'deflate' as a default codec, which is the second breaking change. Fortunately, it's very easy to add back for any API consumers--they can just pass in zlib.[in/de]flateRaw as the codec.
  • The only remaining dependencies for the library are buffer and stream, which have robust polyfills currently available on NPM. Removing these two dependencies, buffer especially, is a more complex undertaking, but I feel better about importing them since they're a core part of the API surface when compared to, say, zlib, which was essentially only imported on behalf of the API consumer.
• Cleans up the codebase now that we're using ES6. Variable declarations have been "un-hoisted" since let statements are lexically scoped, and deprecated new Buffer calls have been completely removed now that Node < 6 no longer needs to be supported.
• Adds a performance optimization for toBuffer which results in a ~30% performance improvement. Overall, this PR seems to be within the regular margin of error for performance, with some tests slightly improving and others slightly worsening, but I found an optimization just to be on the safe side.

[valadaptive]

Did you use a tool to do the translation? I'm curious how difficult it would be to swap the lets for consts where possible.

The translation to ES6 class syntax was done manually. The translation from var to let was done by enabling ESLint's no-var rule, letting ESLint autofix as many cases as possible and manually going over the rest.

There are tools like lebab that can automatically convert to ES6 classes as well, but it seems much less well-maintained than ESLint, so I opted to go with ESLint + the manual approach.

It should be fairly easy to use const where possible--ESLint has a prefer-const rule which has an autofix available.

[mtth]

Thank you very much for this @valadaptive! Follow up release PR: https://github.com/mtth/avsc/pull/428