Add additional grains calls

[amannch]

This pullrequest provides the functionality to manipulate grains data for the minions.
The following calls were added:

Salt:					net-api-client:

salt.modules.grains.set			Grains.set
salt.modules.grains.setval		Grains.setValue
salt.modules.grains.setvals		Grains.setValues
salt.modules.grains.append		Grains.append
salt.modules.grains.delkey		Grains.deleteKey
salt.modules.grains.delval		Grains.deleteValue
salt.modules.grains.remove		Grains.remove

The method signatures try to mimic the corresponding salt call. Optional parameters are represented with the Optional datatype.

Additionally, this PR adds basic unit tests for each new method.

[renner]

Thanks @amannch, your patch looks good to me! From my perspective it is okay to rename the module calls to create a nicer API (e.g. delkey -> deleteKey), do you agree @lucidd?