Split out infer_edges code

infer_edges.c:
- split out code from ctx_infer_edges.c
- added tests: src/tests/infer_edges_tests.c
- made multithreaded

graph_writer.c:
- added fast graph_write_all_kmers() method

.travis.yml:
- moved scripts into travis/*
- only run coverity scan on: linux + gcc + coverity_scan branch

Makefile:
- RECOMPILE=1 forces recompile every time