node_access_write_grants should not be called directly

First of all thanks for the useful module (and sorry for my bad english :P)

as reported from the documentation and comments in code, node_access_write_grants() should not be called directly by a contrib module, should be called node_access_acquire_grants() instead.
Replace the call to node_access_write_grants with node_access_acquire_grants allow better integration with other content access modules and be able to take full advantage of the mechanism provided by the core, including alter the grants (hook_node_access_records_alter() now is completely bypassed).

I also think that in order to maintain the consistency of the nodes configurations data is more correct to read the configuration from the table nodeaccess and not directly from node_access, which is subject to change in case of multiple modules that manage access to content.

patch attached