create generic "oid column" on Table, add late-compiling

[Mike Bayer]

first of all, make Table have an "oid column" that is just, "oid column", and thats it. we dont know what its called until the column is compiled. that way SQL construction totally happens even if it references oid column.

then if the particular engine doesnt have an OID column and the table has no primary key, then raise an exception when you try to compile against it. unless its inside ORDER BY, in which case, just skip it.

that way oid column doesnt complicate SQL construction and its "value" is only calculated at compile time.

[Mike Bayer]

this is implemented in the 0.2 branch.