add file store atomic write
rescue nil on file store read
add file store test for the case that the marshalled data got corrupted
Merge branch 'fix/atomic-write' of github.com:rubemz/sprockets into rubemz-fix/atomic-write

Conflicts:
	lib/sprockets/cache/file_store.rb
	test/test_cache_store.rb
Merge branch 'rubemz-fix/atomic-write' into atomic_write