sqlparse.utils.remove_quotes does not remove backtick

[codenamelxl]

Should back tick ` considered to be a quote character ?

For example `test_schema` when passed to remove_quotes() should return schema name without the backtick. https://github.com/andialbrecht/sqlparse/blob/23d29933ddc4272b495d36e0e32d3eaf0c3ef76d/sqlparse/utils.py#L58

I'm not too sure if adding backtick to this line will break anything. If it's OK to do, I'll send a PR.