parsePeriodExpr accepts lowercased input only, which is not ok for period transactions

gistfile1.txt

First transaction will parse, second will not:
```
cat every-month.journal                                                                                                                                ~/devel/haskell/darcs-get/hledger/examples
~ aug to sep
    assets
    expenses    $1

~ Aug to Sep
    assets
    expenses    $2
```

Demo:
```
 ../bin/hledger-budget bal -f every-month.journal                                                                                                           ~/devel/haskell/darcs-get/hledger/examples
hledger-budget: Failed to parse "Aug to Sep": date parse error ()
```