added a way to set headers with a dictionary and changed how headers are set by default

[josephmancuso]

This PR adds the ability to set headers via a dictionary:

request.header({
    'Cache-Control': 'max-age=3600'
})

[coveralls]

Coverage increased (+0.03%) to 92.118% when pulling 761853d on add-dictionary-for-headers into d36c1fd on develop.