Fix exists?() performance using find_one().

Use find_one() instead of count(), since it's faster on most datastores for large collections. Especially MongoDB, where count() can't use indexes properly.
Fix write concern for mongo 1.8 in spec.
Update readme and query for new mongo client.
Release 0.6.0.
do not normalize to $in operator with an empty array
Merge pull request #26 from wpeterson/perf/exists

Fix exists?() performance using find_one().
Merge branch 'master' of https://github.com/cheald/plucky into cheald-master
Use only instead of fields in #exists?.
Merge branch 'master' of github.com:jnunemaker/plucky

Conflicts:
	lib/plucky/query.rb
Releasing 0.6.1.
Fix merging two bson object ids into an array.

Refs #28
Correctly merge array of object ids and object id and vice versa.

Closes #28
Merge pull request #30 from jnunemaker/fix-bson-id-merging

Fix bson id merging
Merge pull request #29 from paulwalker/empty_array_crit_value_fix

do not normalize to $in operator with an empty array
Release 0.6.2.

* do not normalize empty array to $in
* correctly handle merging of bson object ids in criteria
Allow a paginate query to override the total_entries value, enabling API consumers to potentially cache expensive counting operations.
Merge pull request #31 from kylev/total_entries

Allow a paginate query to override the total_entries
Remove bundle config from repo.
Change rubygems source in gemfile.
Kill ruby debug. Always causes bundler issues.