Use nokogiri

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
```