handle cuda error codes

fix cuda9.1 compile

- fix cuda9.1 compile (remove includ eof device_functions.hpp/ removed with cuda9.1)
- remove NVIDIA Volta gpus for MAC OSX

Also fflush in printer::print_str() if configured to do so

handle all error codes from the cuda api calls.

fix windows compile

solve #440 thanks @true-datura

Fix compat_usleep for WIN32

Makes casting more explicit.

When compiled with VS2017, the negative applied to the uint wait
time is ignored. Fixed by casting first.

add binary checksums for version 2.1.0

conservative NVIDIA auto suggestion

- fix indention

increase version to 2.1.0

Fix cli questions

Be more conservative with the auto suggestion.

- increase bfactor if `smx <= 6`
- limit memory for pascal < GTX1070 to 2GiB
- limt memory for pascal <= GTX1080 to 4GiB

CMake: fix wrong variable name

add psychocrypt's pgp key

Correct list order

Make sure we don't run without a username

update version

- add separate file for developer pgp key's

change `MICROHTTPD_ROOT` to `HWLOC_ROOT` within the hwlock section

thx @maztheman for spotting the bug

Grammar and wording regarding virus protection software

Correct name of optional feature

That's what it's called now: https://i.imgur.com/juZhtyz.png

Extend low_power_mode to do up to 5 cn hashes at a time.

Fix CLI option start + make cli option pool into an added pool with top priority ithinstead of overwrite

add issue and pull request templates

add message to `CUDA_CHECK...` macros

allow to diable UAC dialog

- add macro `CUDA_CHECK_MSG_KERNEL` and `CUDA_CHECK_MSG`
- add suggestion of typicle errors can be solved

option to controll gpu synchronization

add templates with importend information to optimize the support