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 can be greatly simplified:  https://docs.python.org/2/library/functools.html#functools.total_ordering  so we only need to define `__eq__` and one other it it will fill the rest in in terms of those.