Ruby: Add Sequel as SQL Injection Sink

[maikypedia]

This pull request adds sequel library Sinks for SQL Injection. The list of methods that accept raw SQL statements is incomplete, but I haven't included several methods because they are internal. Here is the list in case it is convenient to add them as a sink :

"aliased_expression_sql_append", "array_sql_append", "boolean_constant_sql_append", "case_expression_sql_append", "cast_sql_append", "column_all_sql_append", "complex_expression_sql_append", "constant_sql_append", "delayed_evaluation_sql_append", "function_sql_append", "join_clause_sql_append", "join_using_clause_sql_append", "literal_append", "negative_boolean_constant_sql_append", "ordered_expression_sql_append", "placeholder_literal_string_sql_append", "qualified_identifier_sql_append", "quote_identifier_append", "quote_schema_table_append", "quoted_identifier_append", "subscript_sql_append", "window_sql_append"

Looking forward to your suggestions.

[alexrford]

Looks good, I checked against https://github.com/jeremyevans/sequel/blob/master/doc/security.rdoc and I'm happy with the coverage here. Haven't been able to run performance tests yet, but I'll update when I'm able to do that.

[alexrford]

Performance looks good. There's a merge conflict in Frameworks.qll now - I can re-approve and merge once this is resolved.