Add programmtic interface

I found myself really wanting a programmatic interface to check strings without having to write to files. This is a simple function for doing so.
Rather than do all this with StringIO, I think it would make more sense to change prep_file to accept a string rather than a filelike as input.  Then, the ecpg.check_syntax call in check_file would have to change a bit to read like:
  ecpg.check_syntax(prepped_file.read())
which is better anyway IMO.  But, if that change was made then you could just pass the string straight through to prep_file instead of doing this StringIO stuff.  Thoughts?