v6.0.0

Add optional rounding mode parameter to `toExponential`, `toFixed` and `toPrecision`.
Add a strict mode to disallow imprecise number/Big conversions when `Big.strict = true`.
Add `toNumber` method.
Add `prec` method to round a Big to a specified number of significant digits.
Add version selector to API documentation.
Change `toJSON` to return exponential format.
Remove *big.min.js*.
Remove `Big.version`.
Rename *doc* folder to *docs* to use it as the Github publishing source.
Add legacy API documentation to *docs*.
Add *README* to *perf* directory.
Refactor test suite, and add `toNumber` and `prec` tests.
Update *README*.