Refactor

Short changelog:

* Added `test` command in npm to `npm test`.
* Restricted list of exported files in npm module.
* Separated `set` and `get` methods for those who don’t need both, in order to be able to include only `require('caret-position/get')` (to minify build size).
* Fixed all jshint warnings.
* Added jsdoc annotations.
* Added wrapper providing both explicit and short API (if you mind, I can get back only explicit API).
* Added standalone build with browserify, as well as alias `npm run build`
* Added badges and icons to the readme