[WIP] Fix 32-bit or 64-bit Cygwin platform detection

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Related tickets | #8
| License       | GNU GPL v3

- [x] Added a test for detecting the right architecture
- [ ] Running test under Windows 64 bits platform

This PR implements most of the functionality described in #8, except for an option to set the package architecture, which should not be needed.

Still left to do:

- Run tests using Windows python (for example, on 64-bit Windows that has 32-bit Cygwin)

Makes exception messages long, but mainly makes it hard to test the messages.  I can use the full path if you can recommend a way to test the exception messages.