reduce size of files created by dump_svmlight_file (tests included, pep8/pyflakes pass)

by (1) printing integers as integers and (2) using printf type "g" instead of "e"

    >>> '%.16e' % 98.7 
    '9.8700000000000003e+01' # not very concise!
    >>> '%.16g' % 98.7
    '98.7' # nice!
    >>> '%.16g' % 1.000000000000001
    '1.000000000000001'
    >>> '%.16g' % 1.0000000000000001
    '1'
    >>> '%.16e' % 1.0000000000000001
    '1.0000000000000000e+00' # precision works the same way in g and e