Move fee increment to Account structure.

When a BIP0032 wallet is implemented and multiple address chains can
be supported by a single keystore, the Account structure will
represent a single wallet (and be renamed to reflect that change),
rather than keeping the collection of Account structs as currently
managed by the AccountManager.  In preperation for this, and to remove
a global variable, move the fee increment for created transactions to
this structure.  When setting the fee, look it up from the default
account.