Xraylib wrap

More work is done to wrap xraylib, such as 

a. make class public
b. more overloading operation, eq, lt, gt, ...
c. more doc
d. sub class

we need to discuss the location of element module.
This still is not right:

```
In [1]: import nsls2.constants

In [2]: e = nsls2.constants.Element(5)

In [5]: type(e.cs)
Out[5]: function

```