Fix string escaping for sqlite

SQLite was defaulting to the MySQL backslash-escaped style, but it uses
postgres-style escaping. This is a SQL-injection vulnerability, and
shouldn't be taken lightly (although SQLite is mostly for testing).
Attempt to unify query escaping between dialects

All dialect implementations re-implemented escaping, resulting in
(roughly) the same code being written four times in the codebase. This
moves all of that into SqlString.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	lib/dialects/postgres/query-generator.js
Small bugfixes, push to get travis to rebuild
Merge branch 'master' of github.com:sequelize/sequelize

Conflicts:
	lib/dialects/postgres/query-generator.js
Merge pull request #700 from PiPeep/master

Consolidate escaping into SqlString