Use nokogiri

[swalberg]

Fixes #7

Switched XML parsing to use nokogiri instead of REXML.

Test suite runs in 1s using nokogiri vs 19s using REXML.

x12(master %) $ time RUBYLIB="./lib" ruby ./benchmark.rb

real    0m1.478s
user    0m1.421s
sys 0m0.056s

x12(master %) $ git checkout use_nokogiri
Switched to branch 'use_nokogiri'
x12(use_nokogiri %) $ time RUBYLIB="./lib" ruby ./benchmark.rb

real    0m0.253s
user    0m0.197s
sys 0m0.051s

x12(use_nokogiri) $ time rspec
...............................................................................

Finished in 0.33452 seconds
79 examples, 0 failures

real    0m1.026s
user    0m0.881s
sys 0m0.134s
x12(use_nokogiri) $ git checkout master
Switched to branch 'master'
x12(master %) $ time rspec
.....................................................................

Finished in 18.49 seconds
69 examples, 0 failures

real    0m19.175s
user    0m18.753s
sys 0m0.411s