Cannot override default config values with nil

Unfortunately, due to https://github.com/opscode/chef/issues/1507, users cannot accomplish things like #288 & #289. 

By comparing [our default config entries](https://github.com/edelight/chef-mongodb/blob/master/attributes/dbconfig.rb) with [mongo's default entries](http://docs.mongodb.org/v2.4/reference/configuration-options/), shows that `logpath` is the only entry we set which will be unable to be unset entirely. Every other entry we set has a default value provided by mongo, so unsetting can be approximated by an explicit set to that default.

Our available options include:
1. to check for the `syslog` entry, which would unset `logpath`. If a user actually wants to replicate the behaviour of unsetting `logpath` (without setting `syslog`), the user may use a value of `/dev/null`.
2. to stop providing a default logpath in code, perhaps just documenting it in the readme
3. to accept a non `nil` form of nil. This can't be `false`, though it could concievably be `""` or `0`. I'd probably just go with `"nil"` if we needed this.

Considering this range of risk, I suggest we implement option 1: syslog check. I'd personally prefer 2: nil default, but it is incompatible and I can't think of a way to safely mark it as deprecated.

Any objections to syslog check?