ENH: allow to disable wrapt extensions at run time via WRAPT_DISABLE_EXTENSIONS env var

[yarikoptic]

since #98 drives me nuts and no easy way to workaround happen some user starts hitting it

[coveralls]

Coverage increased (+0.003%) to 93.399% when pulling 84ade58 on yarikoptic:develop into 963866f on GrahamDumpleton:develop.

[coveralls]

Coverage increased (+0.003%) to 93.399% when pulling 09376c7 on yarikoptic:develop into 963866f on GrahamDumpleton:develop.

[yarikoptic]

@GrahamDumpleton could you please consider merging this PR? This so far was the only way for us to get over the hump of #98 , but even when that one is fixed, it might be useful for similar occasions or to provide comparative testing

[GrahamDumpleton]

Note though that dropping all the Travis changes. Not needed as it invokes tox to do the test, which then tests for all combinations.

[GrahamDumpleton]

Stupid Travis. Need to list both 3.5 and 3.6 as neither part of default image, so will not test the other. Always running tests for <3.5 in each case.

[yarikoptic]

ah, now that you pointed me to tox, I saw "WRAPT_EXTENSIONS" so in principle I could have reused it instead of coming up with a new one. Feel free to RF or let me know if I should

[GrahamDumpleton]

WRAPT_EXTENSIONS was install time disabling. If you look at the current code in 'develop' you will see I have renamed that environment variable to WRAPT_INSTALL_EXTENSIONS. Then have separate WRAPT_DISABLE_EXTENSIONS to disable at run time, after installed. I believe I got all the changes correct. If you can check develop branch for me that would be great. Am headed out the door right now, so can't go back and fully verify until tomorrow.

[GrahamDumpleton]

Release in 1.10.11. Let me know if any issues.

[yarikoptic]

thanks! if tests for travis pass in datalad/datalad#1705, means all is kosher for me in this regard ;)