Add command to print detected version of visual studio

Closes #140
Add apm install --check

This will compile a native module to ensure a build toolchain is
successfully detected.
Add index.js to native module
Match process.platform directly
Don't throw error when visual studio isn't installed
:lipstick:
Check for option in argv object
Merge pull request #141 from atom/ks-make-visual-studio-optional

Make visual studio optional
Print python version
Include Visual Studio version on Windows
Remove visual studio command

This is now included in apm -v
Output git version from apm -v
Default git version to empty string
Update spec to expect apm -v to be async
Add missing path require
Add missing fs require
Use options.callback when printing versions
Support disabling magenta
0.70.0
Upgrade to npm v1.4.20 to resolve ELIFECYCLE errors (#142)