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 457b5e0d62c0aeb225887603f742be338578afc7 on matiboy:attachments into d568f4edd326adb451b915ddf66cf1a37820e3ca on linkedin:master.

[coveralls]

Coverage decreased (-0.1%) to 92.121% when pulling 16af359dc418be94e13ffacb4af61eef06c3a189 on matiboy:attachments into d568f4edd326adb451b915ddf66cf1a37820e3ca on linkedin:master.

[coveralls]

Coverage increased (+0.4%) to 92.661% when pulling ba1918adb972e325da27ee217d178495de5064a2 on matiboy:attachments into d568f4edd326adb451b915ddf66cf1a37820e3ca on linkedin:master.

[coveralls]

Coverage increased (+0.4%) to 92.614% when pulling 00eedf6e982dfb8c1daab3c0ea3bb653661eb77e on matiboy:attachments into d568f4edd326adb451b915ddf66cf1a37820e3ca on linkedin:master.