Improved error handling in extractRecipient() and improved recipient documentation

[eladnava]

Closes #186.

Returns an error to the callback in the following circumstances:

1. No valid recipient key was provided
2. A string recipient key (topic, notificationKey) was provided, but was not a string
3. An array recipient key (registrationIds, registrationTokens) was provided, but was not an array
4. A valid recipient key was provided but contained false/null/undefined/empty string

In addition:

• Added better handling for the unsent registration tokens detection, in case of a string recipient (it was not handled properly)
• Added tests to check for recipient errors
• Added tests to check for empty string recipients
• Removed a previously skipped test which has been implemented
• Listed recipient keys and types in README.md along with error-handling behavior

[eladnava]

@hypesystem fyi

[eladnava]

@hypesystem Fixed, thanks!

[hypesystem]

Very good update :+1: good with some better tests. We could decide to leave the considerations in https://github.com/ToothlessGear/node-gcm/pull/190#commitcomment-15202646 for a later PR, if you wish, but it would also fit in here.

[eladnava]

@hypesystem I agree that we should be checking for multiple recipient keys and erroring out in that case.

I think that we should not be using the Array.prototype.foreach() traversal method in extractRecipient(), because we can't return out of the function easily. Therefore, I have reverted to the good old for(var i in array) and will be pushing the commits in a few.

[eladnava]

@hypesystem Implemented the multiple recipient error logic.

[eladnava]

@hypesystem We good to merge? =)

[hypesystem]

Absolutely :smile: