Added test folder, test data generators.

Python scripts for generating test data have been added. There is a
module called `env' that should be imported to resolve directory
structure. All generated data goes into example_data and this folder is
excluded from VC.
Bugfixes for bp_tree.

1) Unpacking of header for internal node was not being done.
2) Keys are arrays of, maybe, null terminated strings. This is now
   properly dealt with.