ISSN title and number

[herrboyer]

Hi,

This PR adds two new methods to pymarc.Record :

• issn : returns the ISSN number stored in 022$a
• issn_title : returns the key title stored in 222$a & $b

Note: I've only added the tests, but not updated the documentation : I just wanted to know your opinion on these before writing it :)

[edsu]

I like it!

[herrboyer]

Great, thanks ! The tests are passing and I've mentioned the ISSN methods in the readme, let me know if I there's anything else I can do.