Adding attachments to event

[matiboy]

1. Allows to add an attachment to an existing event
2. Can take a base64 encoded string of bytes, a file path or a file-like object (anything that implements read and gives bytes)
3. Returns the (unloaded) attachment object
4. Allows to load the details of an attachment from their id
5. Uses a (weak) lazy load since content of file can be quite heavy. Loads name / content only when requested

Lots left to do on the feature:

1. When loading an event, create Exchange2010Attachment objects straight under the event.attahcments array
2. Updating attachments
3. Deleting attachments

Tested on 2.7 and 3.4

Would seek advise on documentation (where to insert examples for this)
Also, would like to use a mock library instead of httprettify if possible; please advise whether that's ok and which library you'd prefer if any.

Note that one of the existing tests is failing for me test_list_events.test_dates_are_in_datetime_format both in p3.4 and p2.7

[coveralls]

Coverage decreased (-0.3%) to 91.926% when pulling 457b5e0 on matiboy:attachments into d568f4e on linkedin:master.

[coveralls]

Coverage decreased (-0.1%) to 92.121% when pulling 16af359 on matiboy:attachments into d568f4e on linkedin:master.

[coveralls]

Coverage increased (+0.4%) to 92.661% when pulling ba1918a on matiboy:attachments into d568f4e on linkedin:master.

[coveralls]

Coverage increased (+0.4%) to 92.614% when pulling 00eedf6 on matiboy:attachments into d568f4e on linkedin:master.