Introduce position_type to denote purpose
Reduce line width to 90 chars max
Strip out the btree_unit_test code; it was just getting in the way.
Change trait name to "index" as "file" is being used to refer to the flat file, not the index to the flat file. Continue development of btree_index. Begin development of test framework for insertions. Compiling clean, but two_index_test asserting in shared_ptr. Now that the interface for insertion is a bit firmer, will add some simple insert tests before debugging the more complex two index case.