Added zonename attribute

[ritwickdsouza]

Solves #61. Please review.

[coveralls]

Coverage remained the same at 100.0% when pulling a569f8f on ritwickdsouza:zonename-attribute into 3112afe on openwisp:master.

[nemesifier]

@ritwickdsouza good!

Please update also the example in the documentation for the OpenWRT backend:

https://github.com/openwisp/netjsonconfig/blob/master/docs/source/backends/openwrt.rst#general-settings-example

The example should reflect the change you just introduced.

(the result is published at http://netjsonconfig.openwisp.org/en/latest/backends/openwrt.html#general-settings).

[coveralls]

Coverage remained the same at 100.0% when pulling c7b3d2b on ritwickdsouza:zonename-attribute into 3112afe on openwisp:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 2061929 on ritwickdsouza:zonename-attribute into 3112afe on openwisp:master.

[nemesifier]

Good job. Thanks.