Issue #157: bean-web crashes on prices that are zero. (blais/beancount)

[Andrew Stein]

New issue 157: bean-web crashes on prices that are zero.
https://bitbucket.org/blais/beancount/issues/157/bean-web-crashes-on-prices-that-are-zero

Andrew Stein:

I have some prices that are zero in the beancount file. For example:
```
2011-09-14 price MCMXX 0 MCMXX
```

`bean-web` crashes when processing these.

This is the stack trace
```
Traceback (most recent call last):
File "/usr/local/bin/bottle.py", line 862, in _handle
return route.call(**args)
File "/usr/local/bin/bottle.py", line 1740, in wrapper
rv = callback(*a, **ka)
File "/usr/local/lib/python3.6/site-packages/beancount-2.0b13-py3.6-macosx-10.12-x86_64.egg/beancount/web/web.py", line 1030, in wrapper
app.price_map = prices.build_price_map(entries)
File "/usr/local/lib/python3.6/site-packages/beancount-2.0b13-py3.6-macosx-10.12-x86_64.egg/beancount/ops/prices.py", line 116, in build_price_map
for (date, rate) in remove_list]
File "/usr/local/lib/python3.6/site-packages/beancount-2.0b13-py3.6-macosx-10.12-x86_64.egg/beancount/ops/prices.py", line 116, in <listcomp>
for (date, rate) in remove_list]
decimal.DivisionByZero: [<class 'decimal.DivisionByZero'>]
127.0.0.1 - - [29/Jan/2017 16:43:05] "GET / HTTP/1.1" 500 741
```