Added #hash implementation for Occi::Core classes

+ added #==, #eql? and #hash for Occi::Collection
Formatting fixes + linking fix

* A Link must have a unique occi.core.id before it
  can be inserted into Links
* Specs should be always separated by at least
  one blank line, for readability
Fixed attribute parsing from JSON

* Hashie::Mash has to be re-typed to Occi::Core::Attributes
  including all its child elements
* DRYed the parser code for JSON
Fixed comparators for Categories and Entities

* Specs are still failing, something is broken in
  the JSON parser
Merge pull request #40 from arax/cmp_fix_2

Updated comparators (#==, #eql? and #hash) for Occi::Core and Occi::Collection
Updated parsers

* Refactored OVA/OVF parser
* Updated error handling
* Simplified XML and JSON parser
Enabled strict XML parsing
Merge pull request #41 from arax/parser_ext

Updated parsers
Added cmp specs for ActionInstance
Added stubs for attributes specs
Merge pull request #42 from arax/cmp_specs

Added some specs for ActionInstance and Attributes