Enable absolute imports and minify distributable

[Tyriar]

Fixes #1314
Fixes #1254

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This PR seems like a good place to start to move to v4 as it solves a bunch of problems and will let us remove the browserify build step as it works in lib/ now. If you want to review alt+click the collapse button in GitHub to collapse all files and then ignore all files inside src/ except for tsconfig.json.

It makes the following high-level changes:

• Enables absolute imports via baseUrl and paths in tsconfig.json for all paths referencing common, core and ui modules:

  // before
  import { DEFAULT_COLOR } from '../../common/Types';
  import { IColorSet } from '../../ui/Types';
  // after
  import { DEFAULT_COLOR } from 'common/Types';
  import { IColorSet } from 'ui/Types';

• Moved the tsc build output to out/, this is no longer suitable to directly import because the absolute paths need to be packaged.
• Webpacks out/ as a production umd module in lib/, this is the new main file in `package.json
• The demo imports from out/ by default but can import from lib/ by just uncommenting a line to verify the packaged portion (it requires you to run yarn package though):

  // Use tsc version (yarn watch)
  import { Terminal } from '../out/public/Terminal';
  // Use webpacked version (yarn package)
  // import { Terminal } from '../lib/xterm';

• Removed gulp and lots of dependencies
• Replaced gulp tests with npm scripts, removed build gulp tasks completely
• Removed coverage reporting as it wasn't helping and can be misleading
• Moved xterm.css into css/, removed tasks for move it around

Follow up changes after this look good:

• Sort imports
• Remove src/addons and all build steps completely
• Update documentation:
  • dist/ -> lib/
  • New css path

Notes:

• While ts-loader is capable of doing the build in a single step, we lose great tsc features like strict null checking and composite projects. Splitting it up like it has always been also makes it much easier to understand imo.

[Tyriar]

Need to figure out how to resolve the paths for the tests.

[Tyriar]

And so it begins.