fix issue#2 (empty dependency installs)

[rbuckheit]

summary

Fixes issue #2 (empty dependency installs cause 500 error). We now return an empty archive if user requests install of a package.json with no dependencies.

testing

• Adds server end-to-end tests for the fixed test case
• Adds server end-to-end tests for the /npm/hash route which was previously untested.
• Adds server end-to-end tests for the DELETE /npm/install route which was previously untested.

[charleswli]

🌴