Add a `phonegap:release:ios` task

Build a signed `.ipa` application package from a phonegap project.

Testing
-------

To test `phonegap:release:ios`, you need:

- A provisioning profile from Apple (`.mobileprovision`), edit
  `phonegap.config.provisioningProfile` to point to the file
- `phonegap.config.data.id` needs to match the app ID in the provisioning
  profile
- A signing certificate imported into your Keychain, edit
  `phonegap.config.certName` to match the name of the certificate as displayed
  by Keychain Access or Apple's `security` command

I have run the tests and seen them pass, unfortunately I don't have
provisioning profile or certificate which I can share.

I've also tested with one of our internal apps.

`grunt build` Output
--------------------

    Running "clean:tasks" (clean) task
    Cleaning tasks...OK

    Running "clean:test" (clean) task
    Cleaning test...OK

    Running "copy:test_fixtures" (copy) task
    Created 2 directories, copied 35 files

    Running "copy:test_cordova" (copy) task
    Copied 1 files

    Running "coffee:tasks" (coffee) task

    Running "coffee:test" (coffee) task

    Running "readme_generator:readme" (readme_generator) task
    >> Getting package info from options...
    >> No need to get package info...
    >> Generating banner to place on the very top...
    >> Generating table of contents, you won't get lost...
    >> Digging the past for release information...
    >> Getting the latest changelog from the changelogs folder...
    Your readme file "README.md" is ready!
    OK

    Done, without errors.

Potential Improvements
----------------------

There's hardly any checking of paths provided as arguments in the `phonegap`
config block. This could be improved.