Add 'system' installation option

[emilkovacev]

Closes #6

• Adds a 'system' option to install qiskit-sys with a system installation of the qiskit-c-api
• Adds a test test_version to ensure that the version of qiskit-c-api matches the version of qiskit-sys (to ensure compatibility). Maybe should be moved into build.rs? Open to thoughts.

[emilkovacev]

According to the official *-sys rust docs (link), sys crates should "probe the current system for libfoo before resorting to building from source." I think it's definitely within scope for me to change this within this PR (and would be fairly straightforward) but I'll wait for review comments before moving forward.