Add flag to BPF() to enable debug prints from the verifier. Use by
passing 0x2 to debug= in the constructor. Add docstring for BPF
constructor.

Also, optimize bpf_prog_load so that by default the kernel is not
asked to print the verbose output. The program will be "loaded" a second
time to capture the verifier failure message.

Fixes: #147
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>