Improve signature method calls.
Improve signature method calls.

This seems better than the call_user_func implementation because:

* There's less magic going on; easier to follow & debug
* IDEs won't complain about the "unused" functions
* The method names now adhere to standard used for all other methods

The changes made here are:

* Removed passing of config to internal signing methods; the methods are using the class property so we don't need to pass this through.
* Use a switch instead of call_user_func to determine correct signature method to call.
* Improved naming of signature-generating methods; (these weren't adhering previously because they were being called magically using the constant strings)
* Initialise signUsingRsaSha1-generated signature as an empty string; we're expecting a string back to getSignature, not a boolean.
* Added method documentation to signature-generating methods.

guzzle/oauth-subscriber,pavkatar/oauth-subscriber