[test] add source code check to travis (pep8, etc)

This helps to catch "style" deviations, such as improper indentation, unused imports, etc.

flake8 is commonly used for this

The goal of this issue is to setup the check in travis with "relaxed" conventions.

Conventions such as line length, single quotes vs double quotes, etc, can be established afterwards.

To diminish the risk of introducing errors at the expense of :"style", code coverage (e.g. #10) should be given priority.

For more on code quality tools see https://github.com/PyCQA.

Things to fix (full report):

qty	err code	description
8	E111	indentation is not a multiple of four
1	E114	indentation is not a multiple of four (comment)
1	E116	unexpected indentation (comment)
18	E201	whitespace after '('
20	E202	whitespace before ')'
10	E203	whitespace before ':'
12	E211	whitespace before '['
15	E221	multiple spaces before operator
3	E225	missing whitespace around operator
1	E227	missing whitespace around bitwise or shift operator
117	E231	missing whitespace after ':'
11	E261	at least two spaces before inline comment
3	E262	inline comment should start with '# '
93	E265	block comment should start with '# '
14	E266	too many leading '#' for block comment
3	E271	multiple spaces after keyword
5	E272	multiple spaces before keyword
72	E302	expected 2 blank lines, found 1
6	E303	too many blank lines (2)
14	E305	expected 2 blank lines after class or function definition, found 1
3	E306	expected 1 blank line before a nested definition, found 0
11	E402	module level import not at top of file
38	E501	line too long (118 > 79 characters)
41	E701	multiple statements on one line (colon)
1	E703	statement ends with a semicolon
4	E712	comparison to True should be 'if cond is True:' or 'if cond:'
7	E731	do not assign a lambda expression, use a def
27	F401	'honeybadgerbft.crypto.threshsig.boldyreva.deserialize1' imported but unused
1	F403	'from ecdsa_ssl import *' used; unable to detect undefined names
1	F405	'KEY' may be undefined, or defined from star imports: ecdsa_ssl
1	F812	list comprehension redefines 't' from line 80
3	F821	undefined name '_combine_and_verify'
12	F841	local variable 'aba' is assigned to but never used
29	W291	trailing whitespace
49	W293	blank line contains whitespace
7	W391	blank line at end of file
1	W604	backticks are deprecated, use 'repr()'

amiller / HoneyBadgerBFT

[test] add source code check to travis (pep8, etc) #33