Add option to SchemaTool for ignoring unsupported tables

I suggest adding a new feature to SchemaTool, which allows you to ignore tables, which contain unsupported column types.

Use case:

• You have a legacy database, or a database that is also shared with another application
• You want to use SchemaTool to speed up development
• The database contains tables which are not used in the Doctrine 2 application, and contain unsupported column types

I've encountered this already a few times myself - Basically if you try to use orm:schema-tool:update with a database that contains tables with unsupported column types, it'll throw an error and you won't be able to use it at all. Because schema tool is extermely convenient when developing, I think it would be very useful to have support for this feature.

Implementation:

I think this should be doable by just changing SchemaTool/SchemaManager so, that SchemaManager would contain an additional method (or flag) which works like createSchema, but ignores tables that cause an exception, and SchemaTool would include a flag for using this instead of the standard approach.

I'm looking into implementing this myself, and will submit a patch if this seems like a reasonable approach.