Support specifying open price ranges with asterisk

This adds support for specifying open price ranges by using asterisk on either end of the range. It duplicates some code, so I'm open for suggestion how to do this cleaner.

It also support "*-" range, which basically means ignore the attribute. I just added that for completeness, it's probably pretty useless in practice and could be removed.

Example:
```ruby
conf.price_ranges = ["*-500", "50-1000", "1000-*"]
```

The current jbrien master supports an open range by specifying a negative range, eg. `1000..0`which I find strange. Another approach would be `1000..Float::INFINITY`.