Multiple option value support (e.g. --custom-header)

[woodb]

Adds support for flags requiring multiple values by accepting option value as a tuple and then passing in the double argument to wkhtmltopdf.

Additionally, this adds tox configuration for testing in multiple versions of Python. Also, added a few things to .gitignore that I found were created during testing but not ignored (like .eggs).

Closes #41
Closes #45
Closes #53

[JazzCore]

Thanks!

Small note - there was 2 PRs with multiple option value support - yours and #40. Since #40 also adds support for repeatable comments (which is harder to implement using your PR) I've used most of the code from #40.

There is a small syntax change in how option is specified in current version - you must place custom header tuple in a list (see example below), but now you can specify more than one custom header.

options = {
    'page-size': 'Letter',
    'margin-top': '0.75in',
    'margin-right': '0.75in',
    'margin-bottom': '0.75in',
    'margin-left': '0.75in',
    'encoding': "UTF-8",
    'custom-header' : [
        ('Accept-Encoding', 'gzip')
    ]
    'cookies': [
        ('cookie-name1', 'cookie-value1'),
        ('cookie-name2', 'cookie-value2'),
    ],
    'no-outline': None
}