[WIP] Fix check for empty file when on python 2

[betatim]

Fixes #75

In python 2 there is no peek(), so implemented our own.

Does someone know why six does not have a compatibility thing for this?

• test the new function

[betatim]

The thing missing from this PR is a good idea how we could have caught this before. My unhappiness with the tests keeps growing :-/

[codecov-io]

Codecov Report

Merging #76 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage    92.6%   92.68%   +0.08%     
==========================================
  Files          12       12              
  Lines         500      506       +6     
==========================================
+ Hits          463      469       +6     
  Misses         37       37

Impacted Files	Coverage Δ
osfclient/utils.py	96.42% <100%> (+0.97%)	⬆️
osfclient/models/storage.py	94% <100%> (ø)	⬆️

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