Add "Small amounts for everyday uses" text in "Secure your wallet"

This pull request recycles dropped text about mobiles:
https://github.com/bitcoin/bitcoin.org/pull/232#issuecomment-23499436

And it makes it instead gives a better analogy that Bitcoin is cash and makes keeping small amounts for everyday uses a general good practice, regardless if it's a mobile, laptop, tablet or else.

This pull request also improves the backup text to mention that backups can be used against theft as long as the wallet is kept encrypted.