readme. add wrecker master branch badge to readme.
Fix tests for mysql, postgres
Show error message if not using addressable value for auto migration
Fix migration tests
Merge pull request #297 from westonplatter/readme-add-wrecker

readme. add wrecker master branch badge to readme.
Allow .Open to reuse existing database connections.

This commit allows you to pass a string or an existing database
connection as the source for gorm. The dialect is still required
because a) there is no common reference to it as far as i know, and
b) gorm allows the dialect to differ from the driver. So, for the sake
of simplicity, you still have to specity the dialect.

This is useful if you have an existing transaction, but still
want to use gorm to format your queries.

This is dependent on the defintion of DB in pkg database/sql having
the field 'dsn', which is the database source, obtained via reflect.
Refactor reuse existing database connection
.Count() should always use ToSnake'd foreign keys.

It looks like gorm always uses the snake form of a column by
convention, as seen by searching DBName in scope.go. These counts
were erroring out without the ToSnake'd foreign keys.

Further, the code for has_many and has_one becomes the same (which
makes sense), so I combined the two cases.