Added a pre-commit hook to check style.

Review: https://reviews.apache.org/r/21439
Enabled whitespace/blank_line rule for cpplint.

Enabled whitespace/blank_line rule for cpplint, which covers the
following:
'Redundant blank line at the start of a code block '
'should be deleted.'
'Redundant blank line at the end of a code block '
'should be deleted.'
'Do not leave a blank line after "public|protected|private:"'
'"class|struct:" should be preceded by a blank line'

Review: https://reviews.apache.org/r/19966