Swiss ESR verification

[kurtkeller]

Added a module for Swiss ESR numbers (reference number on Swiss payment slips).

• With the code in stdnum/ch/esr.py I'm confident. However, I don't know whether another module does compatible verification. I created the verification from scratch using the available online docs from the Swiss payment services provider (SIX) and PostFinance (financial institution).
• With the files docs/stdnum.ch.esr.rst and tests/test_ch_esr.doctest I'm not really sure what they do; I tried to create them by looking at similar files.
• The changes to ChangeLog, README and docs/index.rst are in a separate commit, because I don't know whether you want to do that by yourself to keep things consistent.

I added this module because I started to use qrbill for creating the new standard Swiss payment slips. qrbill uses validators for IBAN validation, but sofar does not do any validation on reference numbers. I intend to change validations over from validators to stdnum, since stdnum has all the required validations, excpet for ESR, which can be added with this pull request.

[arthurdejong]

Hi Kurt,

Thanks for your contribution. I've merged it as a45d4f7. The changes to README, ChangeLog and documentation are more or less automatically done on release.

I've only made a few smaller changes to the code. The compact() function now strips leading zeros to ensure that all equivalent numbers end up as the same value after compact(). I've also rewritten the calc_check_digit() function to be iterative instead of recursive and made the implementation more compact.