Move IP Allocator into sub package and out of the core



Awesome test coverage
```bash
=== RUN TestRegisterNetwork
--- PASS: TestRegisterNetwork (0.00 seconds)
=== RUN TestRegisterTwoNetworks
--- PASS: TestRegisterTwoNetworks (0.00 seconds)
=== RUN TestRegisterNetworkThatExists
--- PASS: TestRegisterNetworkThatExists (0.00 seconds)
=== RUN TestRequestNewIps
--- PASS: TestRequestNewIps (0.00 seconds)
=== RUN TestReleaseIp
--- PASS: TestReleaseIp (0.00 seconds)
=== RUN TestGetReleasedIp
--- PASS: TestGetReleasedIp (0.00 seconds)
=== RUN TestRequesetSpecificIp
--- PASS: TestRequesetSpecificIp (0.00 seconds)
=== RUN TestNonOverlapingNameservers
--- PASS: TestNonOverlapingNameservers (0.00 seconds)
=== RUN TestOverlapingNameservers
--- PASS: TestOverlapingNameservers (0.00 seconds)
=== RUN TestNetworkRange
--- PASS: TestNetworkRange (0.00 seconds)
=== RUN TestConversion
--- PASS: TestConversion (0.00 seconds)
=== RUN TestIPAllocator
--- PASS: TestIPAllocator (0.00 seconds)
=== RUN TestNetworkOverlaps
--- PASS: TestNetworkOverlaps (0.00 seconds)
=== RUN TestCheckRouteOverlaps
--- PASS: TestCheckRouteOverlaps (0.00 seconds)
=== RUN TestCheckNameserverOverlaps
--- PASS: TestCheckNameserverOverlaps (0.00 seconds)
PASS
coverage: 92.6% of statements
ok      github.com/dotcloud/docker/networkdriver/ipallocator    0.016s
```