Add unit tests

[henrinormak]

Adding unit tests for core functionality will help increase the stability of the codebase. Currently, there are no tests, however, ideally tests would cover the following use cases:

• Creating Heimdall instances from all possible sources (generating, public key data, public key modulus/exponent)
• Exporting public key data/modulus/exponent and using the exported result to recreate the instance (partially)
• Encrypting/Decrypting, making sure the latter is only possible with private keys
• Signing/Verifying, making sure signing only works properly with private keys

All tests should work for a variety of Heimdall instances (key sizes), as well as for a variety of inputs (short, long, longer).

[henrinormak]

Some basic tests were added in e7e2b16891f2368b9b64af9f30e35c4643a4c746 These do not cover validating the results, for example by using an alternative crypto implementation to decrypt/verify signatures and data. However, even this coverage is better than nothing.