Add Conan support

[Croydon]

This is split from #647

It only contains the minimum changes to get initial Conan support.

With this changes you can create a Conan package by executing

conan create . benchmark/<version-number>@google/stable

or better: to actual test if the package works, the test_package should be executed with it:

conan create . benchmark/<version-number>@google/stable -tf conan/test_package

(the parameter tf stands for test folder).

If no error appears the created Benchmark Conan package can be consumed by projects locally.

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This does NOT contain any kind of publishing/uploading mechanism with mass building for different configurations/operating systems etc. this should be eventually done via #647

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cc @memsharded @dominichamon

[coveralls]

Coverage remained the same at 89.41% when pulling bf88397 on Croydon:croydon/conan_minimal into 4528c76 on google:master.

[AppVeyorBot]

✅ Build benchmark 1575 completed (commit 29b778268a by @Croydon)

[Croydon]

I have actually reconsidered and removed CONAN_RUN_TESTS and enable_gtest_tests for the sake of simplification. The tests for Benchmark should be continued to run via plain CMake as it is done so far.

Please review this @dominichamon

[dmah42]

Thank you all!