misc changes

Hi sangster,

I'll summarise the proposed changes: 

###Readme
added two lines, it was not clear for me if it was possible to use a file instead of a url.

### Feed definition
added an access to each file definition by file name.

### File definition
added access to aliases so a map between GTFS column names and the internal model column names (aliases)

### source updater
In extract_zip_to_tempfiles added an error in the logger if something happened while extracting the zip file (ex: not a zip file)

While parsing the files I found out that the order is determined by required and optional. For the project I am using this gem I needed to process the files in a different order. I'll pose an example: trip.txt rows have references to calendar, calendar_dates and shapes. If I want to check if the row referenced from trips to a shape exists I need to import shapes into the database first. But shapes is an optional file, so according to the previous scheme I could not do that.
 
With the change introduced in the check_files method I just need to change the order in which I add the files in the feed_definition.

The last change is  fetch_data_set_identifier, basically, it threw an exception when reading a local file and called the method "before".

## By the way!
Thank you for the gem. It's a very nice piece of code and it was very useful for mi.

It would be awesome if there was some more documentation. For example, I had to dive into the code to understand that I was able to create a custom feed definition (I wanted to remove the output_maps or define my custom aliases).  Also, I still don't not know how to introduce a custom parser or if  there is  a way to get the row with the alias names in the handler  instead of the column names defined in the GTFS spec. In fact, what are the alias for?

Regards, Juan.