Bug in parseBase for numbers containing a dot ('0.0079')

``` js
new BN('0.0079', 10) // <BN: ffffb22f>
new BN('0.02', 10) // <BN: ffffff3a>
```

`_init`, `parseBase` or `_parseBase` will not care if a dot is included in a base-10 input, will process it as `('.' - 48) & 0xf`.

I think it should just reject such input?
