support of bigInteger#toString(radix), where radix is between 2 and 36

I used some code from my library:
https://github.com/Yaffle/BigInteger
Update README.md
Merge pull request #18 from Yaffle/master

fix for https://github.com/peterolson/BigInteger.js/issues/17
Added test for toString(radix)
Added examples to `toString` documentation
Minified and republished to npm