eliminate glob imports in SequenceFile, include import of VersionMismatchException
exception cleanup & extension

additional version handling exception
__str__ in addition to Javatastic toString
fixed typo in field accessor
remove magic string 'SEQ' and magic number 3 from header handling

replace with single VERSION_PREFIX and len(VERSION_PREFIX)
use version prefix exception when prefix doesn't match

avoids misleading "version mismatch" errors when actual problem
is file *type* errors (e.g. passing a non-sequencefile!)
bumped version number, updated README

listed contributors in README (based on git history)
added .gitignore file to make further git development cleaner