Fix parsing of quoted identifiers in SHOW CREATE TABLE queries

[andremarianiello]

Fixes #733

[kuba--]

In this case, it's also good to fix parseShowCreate function in parse/show_create.go, because right now we don't split db.table.

[erizocosmico]

@kuba-- in this case SHOW CREATE TABLE ´a.b´ should not be split. Anything inside backticks is an identifier and can't be split. The only reason to have them separated would be a.b or a.b.

[kuba--]

if it's a case (that it should not be split) we can just add a dot ('.'), here:
https://github.com/src-d/go-mysql-server/blob/master/sql/parse/util.go#L136
and problem looks as solved.

[erizocosmico]

@kuba-- nope, because when the identifier is not quoted, the dot is not a valid rune for an identifier.