Allowed specification of build tools

Previously the verisons of avr-gcc etc. that were used for compilation
and linking were all set by default. This commit adds the option to
choose which tools to use via various flags specified in `ino build
--help'.
Moved all build flag arg parsing into build.py

It is unnecessary to clutter environment.py since we don't require these
flags globally.
Minor housekeeping

Made makefile select 'python2' via env for portability. Added vim
.sw[op] files to .gitignore. Added a TODO list.