Add GitHub Search script

Uses GitHub's brand new [code search API](http://developer.github.com/v3/search/#search-code). Example use:

> hubot github search bigdata

```
Searched for "bigdata" and found 22152 results, first 5: 
 - perf.js: https://github.com/gnaeus/lodash-join-plugin/blob/7ab623b9553921e65a377be52da670af296450c1/perf/perf.js
 - zTreeTest.htm: https://github.com/fluagen/analytics/blob/3774a14c41ee36eaebe8bbe89cdf578a053883bc/com.cnebula.analytics.local/deploy/bundles/com.cnebula.analytics.reportserver.instrument/webContent/test/zTreeTest.htm
 - zTreeTest.htm: https://github.com/fluagen/analytics/blob/3774a14c41ee36eaebe8bbe89cdf578a053883bc/com.cnebula.analytics.local/deploy/bundles/com.cnebula.analytics.reportserver/webContent/test/zTreeTest.htm
 - zTreeTest.htm: https://github.com/fluagen/analytics/blob/3774a14c41ee36eaebe8bbe89cdf578a053883bc/com.cnebula.analytics.reportserver/webContent/test/zTreeTest.htm
 - pom.xml: https://github.com/CloudifySource/cloudify-recipes/blob/8f071961933a4eeae33e87bbbfd1ee0ef0b88e86/apps/streaming-bigdata/pom.xml
More: https://github.com/search?q=bigdata&type=Code
```

> hubot github search rails/rails ActiveRecord

```
Searched for "ActiveRecord" in repo rails/rails and found 619 results, first 5: 
 - 4_0_release_notes.md: https://github.com/rails/rails/blob/529156969349cb9f0f51b1b464cbae5daba9889c/guides/source/4_0_release_notes.md
 - active_record_validations.md: https://github.com/rails/rails/blob/20281b7f39fda474684f84ffa6a214ef1e93f3d1/guides/source/active_record_validations.md
 - reflection.rb: https://github.com/rails/rails/blob/fa1e101d5469729c0b21c234bde85f731d4ce6e4/activerecord/lib/active_record/reflection.rb
 - association_scope.rb: https://github.com/rails/rails/blob/fa1e101d5469729c0b21c234bde85f731d4ce6e4/activerecord/lib/active_record/associations/association_scope.rb
 - active_record_querying.md: https://github.com/rails/rails/blob/8ab4656fbc1d1ab691fff7c6f2b131300eaa9589/guides/source/active_record_querying.md
More: https://github.com/search?q=ActiveRecord+repo:rails/rails&type=Code
```