Rename for generality
AlterTableNode gets ExistenceCheck argument
Tests
IF EXISTS support for ALTER and DROP actions
More tests
merge nathanlws: Support for IF EXISTS clause on ALTER

Merge pull request #127 from nathanlws/sql-parser/alter-if-exists
https://github.com/FoundationDB/sql-parser/pull/127
Add gettor
merge nathanlws: Add ConstraintDefinitionNode#getExistenceCheck()

Merge pull request #128 from nathanlws/sql-parser/cdn-get-existence
https://github.com/FoundationDB/sql-parser/pull/128