FEAT: Add random_state option (issue #153)

[QBatista]

Address #153

As a first step, here are some changes adding a random_state option to arma.py (issue #153). If the approach is appropriate, I have already written the code for the other remaining files.

[oyamad]

@QBatista Rebase this on master (or merge master).

[coveralls]

Coverage increased (+0.01%) to 93.543% when pulling 6706bb9 on QBatista:add_random_state_arma into 5095334 on QuantEcon:master.

[mmcky]

@lbui01 is this random state feature going to interfere with any work you're doing on random seed (or is that primarily to support lecture site)?

[mmcky]

thanks @QBatista this is a nice addition to arma

[jstac]

@lbui01 is focused on the lecture site.
@QBatista : many thanks