Adding *~ emacs backup files to .gitignore

Adding comments on possible options for using more elaborate foreign
key and adding redundant columns if desired.

In poll creation table, changing column "id" to "poll_id" so as not to
conflict with column name in public.transaction.

Pointing "asset" columns in ask and bid order placement tables towards
asset_issuance table.

Adding viewsFullTransaction method to add views that join
public.transaction to attachment tables.

Tested the whole thing with testNXT blockchain and works fine.