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

@csrcordeiro actually is possible to compare exactly the same as with operators, check the Javadoc here: http://docs.oracle.com/javase/7/docs/api/java/math/BigDecimal.html#compareTo(java.math.BigDecimal)

You could change your asserts to be like this:
```
assertTrue(bigDecimal.compareTo(new BigDecimal("1")) >= 0);
assertTrue(bigDecimal.compareTo(new BigDecimal("1000")) <= 0);
```
The above code is the same as:
```
assertTrue(number >= 1);
assertTrue(number <= 1000);
```