use relative file paths instead of absolute file paths in compiler output

The current behaviour of booc.exe is to output filenames with there full path. for example:

$ booc.exe test.boo
/home/sparky/scary/test.boo(18,43): BCE0006: 'Foo' is a value type. The 'as' operator can only be used with reference types.

The desired behaviour is to use relative paths:

$ booc.exe test.boo
test.boo(18,43): BCE0006: 'Foo' is a value type. The 'as' operator can only be used with reference types.

This is the way all other compilers and interpreters that I've seen work.