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
Closes #723
Update changelog
Better BigInt support for PostgreSQL.
Added execute() for direct migration per file.
Added: Options containing pre/post/success functions.
Merge pull request #726 from durango/bigint

Better BigInt support for PostgreSQL.
Merge pull request #728 from freezy/execmerge

Make explicit migrations possible.
Updated changelog.
Cleaned up configuration.spec.js and made Sequelize throw an error for a dialect that doesn't exist. Closes #729.
Merge pull request #733 from durango/no-speak-engrish

Cleaned up configuration.spec.js and made Sequelize throw an error for a dialect that doesn't exist. Closes #729.
Merge pull request #727 from durango/logic

Added basic where logic to sequelize closes #321
Update changelog.md
Fix mysql query gen test
Merge master
Merge pull request #725 from janmeier/limitoffset

Closes #723 (consistent handling of limit and offset across dialects)
Strings for .find() should be fixed. Also added support for string primary keys to be found easily. Closes #735