LIBCLOUD-507: Add ex_limits call to return a dictionary or resource limits associated with the account. Examples of resource limits would be the maximum number of instances, volumes, snapshots and networks.

Closes #240.

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Update CHANGES.
LIBCLOUD-506: Added the resource extra attribute mapping to the CloudStack
driver and implemented it for networks, nodes and volumes. I also went ahead
and added a number of additional properties for each of the three resource types.

Closes #239.

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Update CHANGES.