Remove workaround for bad HTML error handling

The issue is now fixed, as demonstrated by this real HTTP calls.

    ➜  dnsimple-nodejs git:(master) curl  -H "X-DNSimple-Token: $AUTH" -H 'Accept: application/json' -H 'Content-Type: application/json' -i \
    >       -X POST -d '{"domain":{"name":"example-1419286541.com"}}' \
    >       https://api.sandbox.dnsimple.com/v1/domains

    HTTP/1.1 201 Created
    Server: nginx
    Date: Mon, 22 Dec 2014 22:16:37 GMT
    Content-Type: application/json; charset=utf-8
    Transfer-Encoding: chunked
    Connection: close
    Status: 201 Created
    Strict-Transport-Security: max-age=631138519
    X-Frame-Options: SAMEORIGIN
    X-XSS-Protection: 1
    X-Content-Type-Options: nosniff
    Access-Control-Allow-Origin: *
    Access-Control-Allow-Headers: Authorization,Accepts,Content-Type,X-DNSimple-Token,X-DNSimple-Domain-Token,X-CSRF-Token,x-requested-with
    Access-Control-Allow-Methods: GET,POST,PUT,DELETE,OPTIONS
    ETag: "f465a22b4c29add25efcdf4306b18919"
    Cache-Control: max-age=0, private, must-revalidate
    X-Request-Id: a9db26f5-9122-4086-a9ca-6f60653de0ff
    X-Runtime: 0.747219
    Strict-Transport-Security: max-age=315360000

    {"domain":{"id":1848,"user_id":null,"registrant_id":null,"name":"example-1419286541.com","unicode_name":"example-1419286541.com","token":"x","state":"hosted","language":null,"lockable":true,"auto_renew":false,"whois_protected":false,"record_count":5,"service_count":0,"expires_on":null,"created_at":"2014-12-22T22:16:36.954Z","updated_at":"2014-12-22T22:16:37.336Z"}}

    ➜  dnsimple-nodejs git:(master) curl  -H "X-DNSimple-Token: $AUTH" -H 'Accept: application/json' -H 'Content-Type: application/json' -i \
    >       -X POST -d '{"domain":{"name":"example-1419286541.com"}}' \
    >       https://api.sandbox.dnsimple.com/v1/domains

    HTTP/1.1 400 Bad Request
    Server: nginx
    Date: Mon, 22 Dec 2014 22:16:46 GMT
    Content-Type: application/json; charset=utf-8
    Transfer-Encoding: chunked
    Connection: close
    Status: 400 Bad Request
    Strict-Transport-Security: max-age=631138519
    X-Frame-Options: SAMEORIGIN
    X-XSS-Protection: 1
    X-Content-Type-Options: nosniff
    Access-Control-Allow-Origin: *
    Access-Control-Allow-Headers: Authorization,Accepts,Content-Type,X-DNSimple-Token,X-DNSimple-Domain-Token,X-CSRF-Token,x-requested-with
    Access-Control-Allow-Methods: GET,POST,PUT,DELETE,OPTIONS
    Cache-Control: no-cache
    X-Request-Id: 14277b35-afc4-4dae-837f-17b9c57983bd
    X-Runtime: 0.103965

    {"message":"Validation failed","errors":{"name":["has already been taken"]}}
Update object responses
Split zone get/import

It makes a little bit easier to understand what's going on. Moreover, they are currently not idempotent, and they will probably take different routes in the future.
Zone#get() has been updated to support also JSON

See http://developer.dnsimple.com/domains/zones/#get
Merge pull request #11 from weppos/cleanup

Split domains.zone get/import, clean up code & readme