simplest usage

I want to provide simplest usage for average users, and detailed usage for advenced users.

So, I want to provide

``` js
var assert = require('power-assert');
assert.ok(true);
```

rather than

``` js
var loader = require('power-assert-loader');
var assert = loader.loadLibrary('assert');
assert.ok(true);
```

pros:
- absolute simplicity
- easy to run with browsers (when we provide browser build)

cons:
- What do you think? @jamestalmage

Start by comparing the proposed `require('power-assert')` usage with the existing `power-assert-loader` and its `loadLibrary('assert')` entry point. Clarify whether the project should support the simpler API and browser builds, then document the accepted usage and define completion around an agreed design.

Written by the indexing model from the issue text.