Ensured that OCCI IDs are always unique in OpenNebula backend

* If available Compute, Network and Storage use the IDs already available in the OpenNebula resource description.
* For links the IDs were added to the OpenNebula compute erb.

#11
fixed commas in OpenNebula compute template

#11
Check if resource with OCCI ID already exists before creating a new resource

* The server will return the HTTP status code 409 Conflict if a resource with the same OCCI ID already exists

#11
Fixed bug, where links where not added to compute resource when parsing compute resources in OpenNebula backend
Gemfile changed to use new occi 2.5.3 gem which fixes problems with link rendering
for the OpenNebula backend in compute changed mixins in links to be strings instead of OCCI::Core::Mixin objects
Improved deploying of resources in OpenNebula

* in the OpenNebula backend when deploying compute resources with OS templates the OCCI ID is now set
* for deploying compute, network and storage all mixins are now stored in the OpenNebula resource description
fixed typos
fixed another typo
fixed mixin retrieval from OpenNebula backend

* changed search path from OCCI_MIXIN to TEMPLATE/OCCI_MIXIN
* used .text method of OpenNebula XMLElement
fixed setting of OCCI_ID for templates in OpenNebula backend
delete OCCI_ID and OCCI_MIXIN from template in OpenNebula backend
Merge pull request #22 from ffeldhaus/iss11

An attempt to fix link rendering and ensure that OCCI IDs are always unique
small fix for parsing fs and swap storage in OpenNebula backend when fs format is not found
Updated readme with instructions on which group the OpenNebula admin user must be in

[ci skip]
in OpenNebula backend check if name exists for resources
Fixed null title values breaking xml parser

Another attempt to fix '"title":null' attributes breaking
XmlSimple when converting compute resources from JSON
to XML.
clarified authentication in README when using OpenNebula backend

[ci skip]
changed version number to 0.5 for HTTP headers sent by rocci-server

[ci skip]
Views: fixed rendering of links in text view