Using generic types to specify column families when opening database...

[iSynaptic]

This change requires no changes to existing tests. Fixes #175 and fixes #177.

[iSynaptic]

@DarkEld3r I hope this change represents a happy medium between the way open_cf has worked in the past and your suggestion in #177. Rather than creating a new constructor, the existing ones should be easier to use. For example:

rust-rocksdb/tests/test_column_family.rs

Lines 112 to 114 in d00e03f

	let options = Options::default();
	let cfs = DB::list_cf(&options, &n).unwrap();
	let db = DB::open_cf(&options, &n, &cfs).unwrap();