Change ledger-next-amount to be case-sensitive

Otherwise if there is an account that's name only has one part, the
regex can match it as a currency and as the beginning of an amount.
E.g. if we have the line "Expenses         45 USD", then the old
ledger-next-amount will jump to Expenses instead of to 45.
Merge pull request #200 from errge/master

 	Change ledger-next-amount to be case-sensitive