Update developer-install.md
Update developer-install.md

Changes:
- Add git to dependencies.
- Restructure Windows section into subsections.
- Add additional tips on getting the install to work on windows.
    - How to get Leiningen to work with Cygwin.
    - Running Cygwin as admin is (may be?) required.
LightTable/LightTable,LightTable/LightTable,LightTable/LightTable