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 is a picky thing, but I would order this as `def __init__(self, element, info_type)`  so that the order is similar between this and the sub class.