obj: use linear allocator for pmemobj operations

Quick-and-dirty solution to test fake obj API using a working
mmap'd-file based allocator.
Merge pull request #9 from pbalcer/fakeobjs

obj: use linear allocator for pmemobj operations
obj: fix style issues

It's my editor fault ;)
Merge pull request #10 from pbalcer/fakeobjs

obj: fix style issues