Enable passing override opts to `mcv.remote.conn_spec`

Prior to this commit, `mcv.remote.conn_spec` would _only_
return default values.  With this commit, if you pass it another
dict, it'll return the merge of the defaults + the given
overrides.  This is a more convenient way to construct connection
settings normally.
Add rudimentary tests for mcv.remote.conn_spec

Although really these are questionable since they're mostly
just testing mcv.util.merge_dicts.
Merge pull request #39 from elliot42/master

Enable passing override opts to `mcv.remote.conn_spec`