[api] RESTful api for Subnet and IpAddress

[anurag-ks]

Implements and closes #10

This is a WIP PR, I wanted to confirm all the endpoints that we would be implementing in django-ipam.

[coveralls]

Pull Request Test Coverage Report for Build 98

• 66 of 66 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 87:	0.0%
Covered Lines:	273
Relevant Lines:	273

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💛 - Coveralls

[anurag-ks]

@nemesisdesign I first wanted to confirm these endpoints are okay or not and then move on to writing tests for these views.

[nemesifier]

@anurag-ks you have to implement CRUD operations as described in http://openwisp.org/gsoc/ideas-2018.html