Avoid crashing in the has_bit_field class method when column does not exist

Add support to avoid crashing when loading a rails environment before the column is defined in the database.  For applications that might load models during a migration this will save their migration from failing before it has a chance to create the necessary columns for the has_bit_field to work.  

Also includes some updates to support running in ruby1.9.3 for rails 3.x apps (running the local test suite)