* Initial work on flag trapping using functions called <flag>_handler and default_flag_handler
* Update packages so they do not obliterate flags
* Added append to EnvironmentModifications class
* changed EnvironmentModifications to have append_flags method
* changed flag_val to be a tuple
* Increased test coverage
* added documentation of flag handling
Package repositories now look like this:
top-level-dir/
repo.yaml
packages/
libelf/
package.py
mpich/
package.py
...
This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).
