raco: move languages into language subpackage

This is somewhat like the refactoring I did to move all the
expressions into the expression subpackage. This time, I'm trying to
consolidate all the languages.
raco: refactor rules and logical algebra

1. Move the myrialang rules that have nothing to do with
Myria per se to the rules file.
2. Move the LogicalAlgebra from algebra.py to language/ package.
3. Create a OptLogicalAlgebra class that is an optimized logical algebra.
catalog: add get_scheme as abstract method
scheme: add a get_names function
sql backend: basic partial implementation and some tests
travis: fix sqlite3 in ci environment
sql.catalog: add a module-level comment
Merge pull request #261 from uwescience/sql-lang

Sql backend: basic, incomplete implementation and some tests