No longer require restart for toggle exchange rate

Fixes issue #643 and allows fiat button to work without restart

No logic is required because Send tab gets removed and added for every
toggle.
abstract and improve seed and key methods
more robust get_seed_text
fix: do not lower xpub
Merge pull request #662 from wozz/issue-643

No longer require restart for toggle exchange rate
lower default fee. fixes #403
simplify getseed: do not return hex
Litecoin: update default servers list
Merge remote-tracking branch 'upstream/master'

Conflicts:
	lib/wallet.py
Litecoin: update more labels and URLs