ENH: Make integer checking function isinteger more robust

The function accepts arrays that have dtype=int.
ENH: Use the resolved plates of the constructor

Previously, the main class Node resolved the plates and assigned that as
the plates of the node. Now, constructor may resolve the plates and
those plates will be used. This allows the constructor to resolve such
plates that depend on some other parameter than just the parents.
DOC: Add docstrings to Dirichlet
ENH: Implement a node class for beta random variables
TST: Add unit tests for Beta node
ENH: Add a node class for binomial random variables
TST: Add unit tests for Binomial node
ENH: Add Beta and Binomial to automatically imported nodes