The "npm" executable can not be found on windows. Using "npm.cmd" instead of "npm" fixes the tests.

 - Added two constants for *nix and windows executables and use the fitting one after running an os detection.
 - Tests adjusted to reflect the npm command string change.

Fix jhipster#312

- hard coded, os specific path string replaced with os-independent version
  in GeneratorService and GeneratorServiceTest
- removed unused imports in GeneratorServiceTest

- extended ApplicationProperties for npm process configuration
- the default value for tempDir in ApplicationProperties is the OS spec. temporary folder
- removed OS detection code from JHipsterService
- OS detection and npm path adjustment done to ApplicationProperties before each test