Adding .xls/xlsx support #42

[levivm]

Added support for .xls and .xlsx format, using xlrd python lib.

[nicokoch]

Two things I came across reviewing your code:

• The comment in your parser describes the method as extracting .docx files :-P
• The code for .xls and .xlsx seem to be very similar (or exactly the same?). Instead of copying the code, it would be a lot cleaner to reuse it (an example of how to do this can be seen in the .jpg, .gif and .png parsers).

So nothing critical, just some suggestions.

[deanmalmgren]

I just took a quick look, too. Thanks for putting this together; looks great! I made some comments above, mostly building off @kokxx's suggestions and accounting for changes I plan to merge in #39, probably sometime next week.

I was a little confused about all the changing of the functional test checksums but I see that all of the non-excel checksums were ultimately changed to the same values in the master branch. Apologies if you received an email about a comment I made; I ultimately figured it out and deleted the comment on github :)

[deanmalmgren]

Excellent; thanks @levivm! This is all merged in now.