Option to use Ox as a SAX handler

[krasnoukhov]

I've run feedjira-benchmarks on this and it definitely looks like Ox makes sax-machine more than 30% faster.

Here are the results:

Rehearsal -----------------------------------------------
feedjira     15.490000   0.070000  15.560000 ( 15.703976)
feedjira-ox  10.530000   0.030000  10.560000 ( 10.675326)
------------------------------------- total: 26.120000sec

                  user     system      total        real
feedjira     15.640000   0.060000  15.700000 ( 15.850888)
feedjira-ox  10.520000   0.030000  10.550000 ( 10.667229)

[krasnoukhov]

We've worked with @ohler55 to make Ox fully compatible with Nokogiri in terms of newlines processing. Now, after a few patches (thanks Peter!) I can claim it is compatible.

So, I have both sax-machine and feedjira suites passing:
https://travis-ci.org/krasnoukhov/sax-machine/builds/27847719
https://travis-ci.org/krasnoukhov/feedjira/builds/27848889

I think this is ready to merge once ox 2.1.2 is released. @pauldix, please let me know what you think.