Add support of DROP INDEX statement

[LuckySting]

Problem

Now it is impossible to drop existed index:

index.drop(connection)

leads to exception:

Unexpected token \'DROP\' : cannot match to any predicted input...\n
SQL: 
DROP INDEX <index_name>

Proposed solution

Added an implementation of DROP INDEX to the dialect, so now it render query as:

ALTER TABLE <table> DROP INDEX <index>;

which is a YQL correct statement.