sql: add GROUP BY support. Closes src-d#52. (src-d#86)

* sql: add AggregationExpression interface.
* sql: add function registry to Catalog.
* sql/expression: add Count and First implementations.
* sql/plan: add GroupBy node.

git: add objects table, closes src-d#54 (src-d#77)

sql: support DESCRIBE table. (src-d#66)

sql/parse: generate sort and limit before project. (src-d#44)

* Sort was being generated after Project,
  so it was not possible to ORDER BY
  by a field not included in SELECT.
* Limit was being executed after the Project too.