Added patch support (closes #9)

Adds an apply_patch function that is just a wrapper for git apply. Also adds PATCHES variable to Pybuild spec, which can be used to define a list of patches. Paths in PATCHES are relative to the files directory (note this differs from the PMS, where full paths need to be specified, as we don't have access to most variables when the mod is loaded).

Note that this supports any type of patch accepted by git apply.