Prerelease of update branch

As requested in #51, this is a prerelease of the contents of the update branch as of 28 June 2020. There is currently no active development on this branch, but it may be more valuable as the master branch.

Crow 0.0.6

• ⬇️ made Crow work with CMake 3.8
• ⬆️ upgraded shipped JSON for Modern C++ to 3.4.0

Crow 0.0.5

• ✨ added event sampling
• ✅ added testing server to mock Sentry
• 📝 added example code
• 🔨 overworked threading

Crow 0.0.4

• ✨ added Log4cplus integration

Crow 0.0.3

• ✨ added functions to manage contexts
• ✨ made installation of termination handler more transparent
• ✨ made client thread-safe
• ✨ added gzip compression for requests
• 🏁 overworked Windows version detection
• 👷 added more compilers to CI
• ✅ added code coverage check

Crow 0.0.2

• 👷 integrated Travis and AppVeyor CI (#1)
• 📝 added Doxygen documentation as GitHub page (#4)
• ✨ added function to retrieve last event id (#8)
• ✨ collecting information on user, machine, operating system, and compiler
• ♿ client now compiles with macOS, Linux, Windows (MinGW and MSVC)
• ➖ removed Sole dependency for UUID generation
• ➖ removed Date dependency for ISO8601 date string generation
• ✅ adding unit tests and live test client

Crow 0.0.1

Initial release.