Add support for several more submodule functions

[leftink]

Add support for:

• Initializing a submodule
• Synchronizing the repo with updates to the config
• Reloading the submodule information
• Add the submodule HEAD to the index
• Retrieving the submodule status

Add property for:

• Getting the OID in the index
• Getting the OID in the working directory
• Setting the submodule URL
• Setting the submodule Branch
• Getting/Setting the submodule ignore rule
• Getting/Setting the submodule fetch/recurse rule
• Getting/Setting the submodule update rule

[jdavid]

Looks good, can you please add the unit tests?