Fixes EGI-FCTF/rOCCI-core#10, attrs in headers

Added support for Rack-compliant HTTP headers in Occi::Parser.
The following representations are now supported:
* X-OCCI-Attribute
* X_OCCI_ATTRIBUTE
* HTTP_X_OCCI_ATTRIBUTE
Merge pull request #11 from arax/header_attr_fix

Fixes EGI-FCTF/rOCCI-core#10, attrs in headers