Support Python 2.7 & 3.4+, drop EOL versions

[hugovk]

Fixes #74.

Here's the pip installs for bigfloat from PyPI for the last year:

$ pypinfo -d 365 --percent --pip bigfloat pyversion
python_version percent download_count
-------------- ------- --------------
2.7              63.2%          3,047
3.5              17.9%            865
3.6              13.9%            668
3.4               4.8%            230
3.3               0.1%              5
2.6               0.1%              4

[mdickinson]

Looks very good to me. Thank you!

There's just one missed 3.2 -> 3.4 change that I spotted. I'll fix that and then merge.

[mdickinson]

Looks like the 3.2 should be updated to 3.4 here.