ENH: ujson better handling of very large and very small numbers, throw ValueError for bad double_precision arg #4042
closes #4042

This makes ujson handle very big and very small numbers a bit better, it doesn't help with precision but it should at least be able to handle very small and large exponentials now:

```
In [4]: from pandas.json import dumps

In [5]: dumps(1e-5)
Out[5]: '0.00001'

In [6]: dumps(1e-6)
Out[6]: '0.000001'

In [7]: dumps(1e-7)
Out[7]: '0.0000001'

In [8]: dumps(1e-8)
Out[8]: '0.00000001'

In [9]: dumps(1e-9)
Out[9]: '0.000000001'

In [10]: dumps(1e-10)
Out[10]: '0.0000000001'

In [11]: dumps(1e-11)
Out[11]: '0.0'

In [12]: dumps(1e-11, double_precision=15)
Out[12]: '0.00000000001'

In [13]: dumps(1e-12, double_precision=15)
Out[13]: '0.000000000001'

In [14]: dumps(1e-13, double_precision=15)
Out[14]: '0.0000000000001'

In [15]: dumps(1e-14, double_precision=15)
Out[15]: '0.00000000000001'

In [16]: dumps(1e-15, double_precision=15)
Out[16]: '0.000000000000001'

In [17]: dumps(1e-16, double_precision=15)
Out[17]: '1e-16'

In [18]: dumps(1e-16)
Out[18]: '1e-16'

In [19]: dumps(1e-17)
Out[19]: '1e-17'

In [20]: dumps(1e-40)
Out[20]: '1e-40'

In [21]: dumps(1e-100)
Out[21]: '1e-100'

In [22]: dumps(1e-400)
Out[22]: '0.0'

In [28]: dumps(1e40)
Out[28]: '1e+40'

In [29]: dumps(1e100)
Out[29]: '1e+100'

In [30]: dumps(1e400)
Out[30]: 'null'

In [31]: from pandas.json import loads

In [32]: loads(dumps(1e100))
Out[32]: 1e+100

In [33]: loads(dumps(1e40))
Out[33]: 1e+40

In [34]: loads(dumps(1e-40))
Out[34]: 1e-40
```

I have also modified it to throw a `ValueError` when a bad value is given for `double_precision`:

```
In [25]: dumps(1e-400, double_precision=-1)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-25-e15fa4642646> in <module>()
----> 1 dumps(1e-400, double_precision=-1)

ValueError: Invalid value '-1' for option 'double_precision', max is '15'

In [26]: dumps(1e-400, double_precision=16)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-26-ab74b2f14c7f> in <module>()
----> 1 dumps(1e-400, double_precision=16)

ValueError: Invalid value '16' for option 'double_precision', max is '15'
```

Tested on Python 2.7 on Arch-64. T'would be great if someone could test this out on windows.
