Use os.TempDir()

Use built-in function to get temporary directory path.
Merge pull request #2 from snipsnipsnip/use-builtin-tempdir

Use built-in os.TempDir() for Windows environment
Removed "fmt" from the template, and added it to the inferred packages set, "pkgsToImport".
Used path.Join instead of concatenating paths manually.
	modified:   eval/eval.go