Replaced limit by length and start by offset to remove a possible ambiguity

When using these methods (Although I was really using just get_jobs):

    def get_job_ids(self, start=0, limit=-1):
        """Returns a slice of job IDs in the queue."""

    def get_jobs(self, start=0, limit=-1):
        """Returns a slice of jobs in the queue."""

I stumbled a cross a bug in my application and then I found that my mistake happened when I wrongly interpreted the limit argument of the functions.

Then I notice there's some ambiguity in this case. get_jobs functions `start` parameter can make method seems like it works like works redis `LRANGE` command, so limit is like STOP

http://redis.io/commands/lrange

But is not.

Then I though it can work like python slice notation:

http://stackoverflow.com/questions/509211/the-python-slice-notation
http://docs.python.org/2/tutorial/introduction.html

So limit is like `:end`
But it not also.

Then I noticed it somehow resembles php's `array_slice` notation. 

http://php.net/manual/pt_BR/function.array-slice.php

In my humble opinion, `limit` is a little bit ambiguous, it could easily be mistaken with STOP of redis command or "end" of python slice notation. 

Maybe `length` is a better parameter name.

I also changed `start` paramater to `offset` for the same reasons.

PS: I know get_jobs is no exactly the `array_slice` of PHP since the method would have to be like 

   def get_job_ids(self, offset=0, length=None):

And that would add a lot of problems that doesn't worth mentioning here. If you want, we can discuss this in a another pull request.