Big decimal range

Hi Fixture Factory community,

This pull request is regarding issue #61.
To Implement this, I had to change both the BigDecimal and BigInteger features but I believe it will be ok since I was able to write tests for both cases. I also noticed a little typo in README tutorial so I fixed it.

There are tests failing in master branch but I couldn't figure out what it was about so I didn't fixed it, the tests are:
* shouldCreateImmutableObjectUsingCorrectPartialConstructor
* shouldCreateImmutableObjectUsingAnotherPartialConstructor