add benchmarks using different sax parsers

[kaero]

candidates:

• sax-js
• libexpat binding
• easysax
• node-xml-lite
• ltx
• node-xml

[kaero]

Sanity benchmark has been added for supported backends (sax-js and node-expat for now).

[kaero]

Benchmark compute number of completed xamel.parse calls per second for each supported backend.

$ node ./test/bench/index.js
sax: 2554 ops/sec, 10 test runs
expat: 6321 ops/sec, 10 test runs
done.