Glean contrib

Fixes #7

@johndbritton here is an initial Glean Contrib for [JerseyScripts](http://www.meetup.com/jerseyscript/events/163057292/).  Glean Contrib "eats its own dogfood"; `Glean::Dataset` loads [glean/glean-contrib](https://github.com/glean/glean-contrib) sources. 

```                                                                               
$ glean search --contrib galaxy
lagalaxy/trophies -- lagalaxy trophies: Major trophies won by the LA Galaxy
```
   
```                                                                         
$ glean search --contrib foobar
```         

Core search isn't changed.
```
$ glean search states
us-states -- The United States of America
```                                                                      

Your existing code **just works™**. :smile: 

```
$ glean export lagalaxy/trophies --format=json
{"competition":"MLS Supporters' Shield","year":1998}
{"competition":"CONCACAF Champions' League","year":2000}
{"competition":"Lamar Hunt U.S. Open Cup","year":2001}
{"competition":"MLS Cup","year":2002}
{"competition":"MLS Supporters' Shield","year":2002}
{"competition":"Lamar Hunt U.S. Open Cup","year":2005}
{"competition":"MLS Cup","year":2005}
{"competition":"MLS Supporters' Shield","year":2010}
{"competition":"MLS Cup","year":2011}
{"competition":"MLS Supporters' Shield","year":2011}
{"competition":"MLS Cup","year":2012}
```
 
```
$ glean export lagalaxy/trophies --format=yaml
--- !ruby/hash:Hashie::Mash
competition: MLS Supporters' Shield
year: 1998
--- !ruby/hash:Hashie::Mash
competition: CONCACAF Champions' League
year: 2000
--- !ruby/hash:Hashie::Mash
competition: Lamar Hunt U.S. Open Cup
year: 2001
--- !ruby/hash:Hashie::Mash
competition: MLS Cup
year: 2002
--- !ruby/hash:Hashie::Mash
competition: MLS Supporters' Shield
year: 2002
--- !ruby/hash:Hashie::Mash
competition: Lamar Hunt U.S. Open Cup
year: 2005
--- !ruby/hash:Hashie::Mash
competition: MLS Cup
year: 2005
--- !ruby/hash:Hashie::Mash
competition: MLS Supporters' Shield
year: 2010
--- !ruby/hash:Hashie::Mash
competition: MLS Cup
year: 2011
--- !ruby/hash:Hashie::Mash
competition: MLS Supporters' Shield
year: 2011
--- !ruby/hash:Hashie::Mash
competition: MLS Cup
year: 2012
```