improvements

hello, I have made few changes described in the commits
would you consider merging upstream? please let me know if i can improve it.
the main changes are:
* listeners are cleared in constructor and this avoids returning old request's results
* added options in constructor:
```
var metaInspector = require('node-metainspector');
var client = new metaInspector(siteUrl, { limit: 1024 * 1000 });  // 1mb
``` 
for now options can limit the sized of data transferred with the request object. this avoids downloading tons of data if an url to big files is given.
if you agree i will add documentation to readme.md in the next pull req