WIP: Use user supplied TypeScript installation

[marvinhagemeister]

Instead of shipping our own version of TypeScript we use the one supplied by the user via peerDependencies. I obviously need to break the commit apart and make the history a bit cleaner, but functionality-wise everything works as expected.

Changes:

• use user supplied TS (let the user decide which version he/she wants to use)
• use TS helpers instead of custom ones
• don't force es2015 target
• remove Object.assign polyfill
• upgrade dependencies
• npm script prepublish -> prepublishOnly

[stakx]

Perhaps I am misunderstanding, but based on tslib's installation instructions (reproduced below) I believe that "tslib": "^1.9.0" would require "typescript": "^2.3.3".

# TypeScript 2.3.3 or later
npm install --save tslib

# TypeScript 2.3.2 or earlier
npm install --save tslib@1.6.1

I think it would be fair by now to just require "typescript": "^2.3.3".