Mention about bob.io.base.peek in user guide.

Fixes #24
12 jobs for 24 in 14 minutes and 32 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build
passed #72499
conda-linux
build_linux_27

00:06:44

passed #72501
conda-linux
build_linux_35

00:08:10

passed #72503
conda-linux
build_linux_36

00:07:01

passed #72504
conda-macosx
build_macosx_27

00:03:46

passed #72502
conda-macosx
build_macosx_35

00:03:44

passed #72500
conda-macosx
build_macosx_36

00:03:44

 
  Test
passed #72509
conda-linux
test_linux_27

00:01:53

72.0%
passed #72507
conda-linux
test_linux_35

00:03:01

72.0%
passed #72505
conda-linux
test_linux_36

00:04:22

72.0%
passed #72506
conda-macosx
test_macosx_27

00:01:44

72.0%
passed #72508
conda-macosx
test_macosx_35

00:01:42

72.0%
passed #72510
conda-macosx
test_macosx_36

00:01:43

72.0%