refactor(plugins): DO NOT MERGE server-side support for new plugins API 

Changes include:
* server adjusted to the new plugins API,
* `trader` moved to `tmpName`,
* _Singleton_ changed to simple `exports`,
* trade exchange balance and currency code removed, vel:
* limit orders changed to market orders,
* all `tmpName` access now proxied through methods,
* methods grouped to improve readability.

TODO:
* think about better name for `tmpName` (`pluginManager` or just `plugins`, maybe?¹),
* Bitstamp support for market orders (vel lack of `rate:` property in `opts`).

Breaks:
* compatibility with old plugins.

---
¹ exemplary calls:
```javascript
// plugins
plugins.sendBitcoins()
plugins.trade()
plugins.fiatBalance()

// pluginManager
pluginManager.sendBitcoins()
pluginManager.trade()
pluginManager.fiatBalance()
```
It tracks content changes and just renames the file, instead of losing the diff between the old file and the new file.