Filtering server products based on the region context.
Allow for null snapshot source disks

Google SDK docs say:
"The source disk used to create this snapshot. Once the source disk
 has been deleted from the system, this field will be cleared, and
will not be set even if a disk with the same name has been re-created."
Setting VM IP addresses.
Check for null source image in toVirtualMachine()
Only return data centers in the requested region

Copied the cache handling strategy from AWS driver
Fix Firewall providerVlanId
Don't set firewall ports for ICMP rules

Else GCE complains
Compute instances were cached across accounts

It was not possible to have two GCE accounts loaded due to the cache
conflicts.
Merge pull request #25 from erik-johnson/2014.03

Filtering server products based on the region context.
Merge pull request #26 from labisso/null-checks

Fix NPEs
Merge pull request #27 from labisso/list-datacenters-by-region

Only return data centers in the requested region
Merge pull request #28 from labisso/firewall-network-ids

Fix Firewall providerVlanId
Merge pull request #30 from labisso/firewall-icmp-rules

Don't set firewall ports for ICMP rules - 2014.03
Merge pull request #32 from labisso/compute-cache

Compute instances were cached across accounts - 2014.03
Some bug fixes

And prep for region filter on instances.
Merge pull request #34 from andylyall/2014.03

Some bug fixes
Fix firewall rule ID generation

All rules in a firewall were getting the same rule ID due to an
order of operations bug.
Merge pull request #37 from labisso/firewall-rules

Fix firewall rule ID generation
Fix status bug on volumes
Merge branch '2014.03' of https://github.com/greese/dasein-cloud-google into 2014.03