Default to projectHome instead of .atom/packages.
Add some specs for the default path.
Update contributing file
Fall back to ${ATOM_REPOS_HOME} or ${HOME}/github.

For consistency with apm develop.
Merge pull request #16 from smashwilson/default-to-project-home

Default to project home
Use fs-plus to get home directory
Create specs for generating packages in dev mode.
Use the createInDevMode setting during linking.

Default it to `false` based on our discussions.