Fix #4: Allow osclass within osmatch, from Nmap 6.00
Update NSE script element parsing

1. Add parsing of NSE scripts' XML output. Structured output is built
into a data structure recursively.

2. Fix a warning: Use of uninitialized value in lc (in `_get_ports`)

2. Document `prescripts` and `postscripts` methods of
`Nmap::Parser::Session`

3. Add appropriate tests for script functionality.
Merge pull request #6 from bonsaiviking/update-scripts

Update scripts