Fix module build, add TS declaration files, fix search addon

[Tyriar]

This introduces a breaking change to importing the project, the README file has been updated to reflect this. It's now exporting an ES6-style module which can be imported without the use of require. This was done because I couldn't get the build to work without making this change and because it's moving towards the future. You can still require using regular JavaScript, but there's a Terminal object on the default export.

src/xterm.ts still lives on but it is only the entrypoint for browserify now. This is mentioned in the header.

Fixes #865
Fixes #866
Fixes #867

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Verified core lib and search addon works in demo on Chrome.

Verified core lib and search addon in VS Code:

// Old (legacy import):
import Terminal = require('xterm');

// New:
import { Terminal } from 'xterm';

Verified TS Server picks up declarations/typings using a simple TS file:

[coveralls]

Coverage decreased (-0.02%) to 71.465% when pulling f21d5f2 on Tyriar:865_fix_module_build into acefa1a on sourcelair:v3.

[parisk]

We should also update the docs.