instrument: make the task less verbose by default

Right now, running 'grunt instrument' will output something like

  $ grunt instrument
  Running "instrument" task
  OK
  OK
  OK
  OK

  Done, without errors.

The repeated 'OK' lines seem unnecessary and uninformative to me. They
now only appear when Grunt is run with --verbose.
instrument: use grunt.file.read instead of fs.readFile

This gives output consistent with other Grunt tasks ("Reading
foo/bar.js" instead of "instrument from foo/bar.js").

It also simplifies the code.
instrument: use grunt.file.write instead of fs.writeFile

This makes the output more consistent with other Grunt tasks:

  Writing foo/bar.js...OK

with a green "OK" instead of:

  instrument to foo/bar.js

This also ensures that no files are actually written if the --no-write
flag is passed to Grunt.
instrument: use grunt.file.exists for simpler code
tasks: do not write flags twice

When I run Grunt with --verbose, I already see the flags printed.
Printing them again gives duplicate output:

  Running "instrument" task
  Verifying property instrument.files exists in config...OK
  Options: basePath="coverage/instrumented", flatten=false, lazy
  Options: basePath="coverage/instrumented", flatten=false, lazy

  Done, without errors.
instrument: make low-level messages debug messages
instrument: output number of files instrumented at end of task

This show the number of files instrumented and the number of skipped
files if options.lazy is true:

  $ grunt instrument
  Running "instrument" task
  Instrumented 2 files (skipped 9 files)

  Done, without errors.

This output is similar to the output from grunt-contrib-copy.
storeCoverage: use grunt.file.write to respect --no-write

This makes the code simpler and makes the task not actually write
anything if the --no-write flag has been passed to Grunt.
makeReport: use grunt.readJSON for simplicity
flowEnd: do not print unnecessary OK

The instrument task now print a summary line and the "OK" at the end
of each file can be removed. Verbose output now looks like:

  Running "instrument" task
  Verifying property instrument.files exists in config...OK
  Options: basePath="coverage/instrumented", flatten=false, lazy
  Reading app/js/app.js...OK
  Writing coverage/instrumented/app/js/app.js...OK
  Reading app/js/auth.js...OK
  Writing coverage/instrumented/app/js/auth.js...OK
  Instrumented 2 files (skipped 9 files)

  Done, without errors.

and normal output looks like:

  Running "instrument" task
  Instrumented 2 files (skipped 9 files)

  Done, without errors.
Merge pull request #30 from mgeisler/standardize-output

Standardize output and use standard FS functions