Address some of Issue #16

Address some of Issue #16

* Introduce Function object in bpf.py
 - prog load types (kprobe, socket, etc.) are independent
 - fd is a member of each function, to be used later (e.g. tail call table)
* Allow functions to be defined inline in the .py as a text argument
* Rename BPFProgram to BPFModule, which should make more sense

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