Add eachdist and move tox to root folder (open-telemetry#29)

Move tox and other configuration files to root folder. Add eachdist to make it
easier to handle operations with different packages.

The eachdist is taken from opentelemetry but slighty modified to avoid ignoring
a given path.

Author: mauriciovasquezbernal

Diff for: .coveragerc

@@ -0,0 +1,6 @@
+[run]
+omit =
+    */tests/*
+    */setup.py
+    */gen/*
+    reference/*

Diff for: .flake8

@@ -0,0 +1,18 @@
+[flake8]
+ignore =
+  E501  # line too long, defer to black
+  F401  # unused import, defer to pylint
+  W503  # allow line breaks after binary ops, not after
+  E203  # allow whitespace before ':' (https://github.com/psf/black#slices)
+exclude =
+  .bzr
+  .git
+  .hg
+  .svn
+  .tox
+  CVS
+  .venv*/
+  venv*/
+  target
+  reference/
+  __pycache__

Diff for: .isort.cfg

@@ -0,0 +1,18 @@
+[settings]
+include_trailing_comma=True
+force_grid_wrap=0
+use_parentheses=True
+line_length=79
+
+; 3 stands for Vertical Hanging Indent, e.g.
+; from third_party import (
+;     lib1,
+;     lib2,
+;     lib3,
+; )
+; docs: https://github.com/timothycrosley/isort#multi-line-output-modes
+multi_line_output=3
+skip=target
+skip_glob=**/gen/*,.venv*/*,venv*/*,reference*/*
+known_first_party=opentelemetry
+known_third_party=psutil,pytest