allow to customize locale's dayOffsets

Enable parsing French days:

```
import parsedatetime as pdt
        
class pdtLocale_fr(pdt.pdt_locales.pdtLocale_icu):
    def __init__(self):
        pdt.pdt_locales.pdtLocale_icu.__init__(self, localeID='fr_FR')
        self.dayOffsets.update({u"aujourd'hui": 0, u'demain': 1, u'hier': -1})
 
pdt.pdtLocales['fr_FR'] = pdtLocale_fr
 
calendar = pdt.Calendar(pdt.Constants(localeID='fr_FR', usePyICU=False))
parse_fr = calendar.parse
for date_string in [u"Aujourd'hui", "3 juillet", u"4 Août", u"Hier",
                    u"au jour de hui", u"aujour-d’hui",
                    u"au-jour-d’hui", "demain", "hier", 
                    u"today", "tomorrow", "yesterday"]:
    tt, success = parse_fr(date_string)
    if success:
       print(date_string, tt[:3])
```

Related: [Parse French date in python](http://stackoverflow.com/q/26294333/4279).