Add unsigned support to integer, decimal, and float types.

Remove 'fixed' from all columns and make it present only on string
columns.
Add unsigned support to MySQL

integer, biginteger, float and decimal types can now be unsigned in
MySQL. Both the schema reflection and generation will handle unsigned
values.
Add unsigned support to SQLite.

integer, biginteger, float and decimal types can now be unsigned in
Sqlite. Both the schema reflection and generation will handle unsigned
values.
Update tests for postgres.

It doesn't support unsigned values, so tough luck.
Merge pull request #2383 from markstory/3.0-unsigned-int

3.0 - Add Unsigned int