Add __str__ on TProtocolException

[aisk]

Add __str__ on TProtocolException with same behavior like TApplicationException.

close #104

[codecov]

Codecov Report

Merging #109 into master will decrease coverage by 0.26%.
The diff coverage is 6.66%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
- Coverage   79.95%   79.69%   -0.27%     
==========================================
  Files          43       43              
  Lines        3887     3901      +14     
==========================================
+ Hits         3108     3109       +1     
- Misses        779      792      +13

Impacted Files	Coverage Δ
thriftpy2/protocol/exc.py	40% <6.66%> (-41.82%)	⬇️

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[ethe]

LGTM, thank you!