expose selector JSONFromURLWithString:method:params:orBodyString:completion:
replace add with set because it needs to be possible to override the default headers

when using "add", the headers are merely concatenated anyway. This can easily be achieved still & therefore this change does not remove any ability
resolves #97
Merge pull request #96 from BillinghamJ/issue-#96

[Req] Expose JSONFromURLWithString in JSONHTTPClient
Merge pull request #97 from BillinghamJ/issue-#97

[Req] Expose method for replacing all headers
Merge branch 'master' into issue-#95