Remove use of eval

[jechols]

This is extremely dangerous when processing field settings from untrusted sources: https://github.com/search5/solrpy/blob/master/solr/core.py#L1116

For Open ONI, we're considering just forking the repo to avoid this problem. A simple prototype has verified that it's trivial to construct a "value" which can run arbitrary code before the key check occurs. Given how often data is going to be stored from unknown sources, this just isn't an option for us.