Fix signature saving at Predict saving

[chenmoneygithub]

resolve #1045

Change summary:

• Add a dump_state/load_state pair to Signature class for better modularization.
• Remove the save_verbose option, as it doesn't make sense to change input fields to output fields at loading time. As we are doing "weights-only" saving, the signatures' architecture should match, i.e., input fields and output fields' number and name. Only changeable attributes should be included with saving.
• Save prefix for every field as requested in #1045.
• Added unit testing.