Polish language support and some configuration possibilities

Hi. 
I'm glad to use your library and hope you would like my contribution.
I did extend the library by providing my native language support.
First commit is responsible for that.
You may also like my new features I added later (it's mentioned in README.md):

Sometimes may get quite long results

	humanizeDuration(465472457557)            // "14 years, 8 months, 4 weeks, 2 days, 9 hours, 54 minutes, 17 seconds, 557 milliseconds"
	
	humanizeDuration.maxUnits = 3             // set default max length in units
	humanizeDuration(465472457557)            // "14 years, 8 months, 4 weeks"

You may do not like weeks so you can also
	
	humanizeDuration(9531000000)			// "3 months, 2 weeks, 5 days"
	humanizeDuration.useWeeks = false;
	humanizeDuration(9531000000)			// "3 months, 19 days"
	

Have a nice day.
Michał Janiec