Add vendored code, strip on go build, fix go vet

[tonylambiris]

Hi!

A few changes here:

• Created a vendor directory using dep
• Fix issue with go vet:

  Validating with gofmtValidating with go vet# github.com/tigrawap/slit/logging
  logging/logging.go:25: missing ... in args forwarded to print-like function
  make: *** [Makefile:42: test] Error 2
  zsh: exit 2     make

• Add ldflags to go build for stripping binary symbols (file sizes without and with, respectively):

  ➜ ll bin/slit
  -rwxr-xr-x 1 tlambiris users 3.1M Oct 13 07:12 bin/slit*
  ➜ ll bin/slit
  -rwxr-xr-x 1 tlambiris users 2.2M Oct 13 07:13 bin/slit*

• Changed debug logging to use go's os.TempDir() (my $TMPDIR is personally set to /var/tmp)

[tigrawap]

This is great, thank you!