Vendoring ast-decompiler

Ideally, as a testing utility, we should avoid potential library
collisions. Since it's entirely possible that at project that is
using Specter to test will also use something named ast_decompiler,
I believe the best option for Specter is to vendor this dependency.

Author: jmvrbanac

.coveragerc

@@ -1,4 +1,4 @@
 [run]
 branch = True
 source = coveralls,specter
-omit = tests/*, *pyevents*, *unittest*, *pynsive*, *six*, *python2*, *python3*, *pypy*
+omit = tests/*, specter/vendor/*, *pyevents*, *unittest*, *pynsive*, *six*, *python2*, *python3*, *pypy*

specter/vendor/__init__.py

@@ -12,4 +12,4 @@ commands = flake8
 
 [flake8]
 ignore = H301,H405,H702
-exclude = .tox,docs,build,setup.py
+exclude = .tox,docs,build,setup.py,specter/vendor/*