Make queries that identify which species meet the queried criteria

[cboettig]

Suggested in #49. In general this is difficult; particularly as it is not clear that arbitrary SQL queries should be supported. Ultimately be addressed by redesigning the API to return a few very large joined tables, making subsetting on arbitrary queries both easier and more natural. Meanwhile, most queries not constructed around query species are more-or-less out of scope for the current implementation, and such users should be directed to request direct SQL access instead.

[cboettig]

queries on arbitrary fields are possible in 3.0. Call any function with no species list to return the full table, which can clearly be filtered at will.