Add support for multiple key bags, incl. rework of test files

[nitram509]

This PR supersedes
#63 "Implement DecodeChains function that accepts multiple key bags"
and #64 "refactor tests to use static files, to ease development"
Resolves #62

It contains three main changes/enhancements

• new function DecodeChains() supports decoding multiple safebags with private keys
• refactored tests to use embed feature of go to ease test file creation and usage
• splitting source files to separate pfx encoder and decoder logic, makes code more readable
• fixes a bug in Implement DecodeChains function that accepts multiple key bags #63, when CA certificate chains contained duplicate certificates
• improved documentation for users, to better guide, when to use what function (e.g. DecodeChain vs. DecodeTruststore)

Any feedback welcome.

[nitram509]

Hi @AGWA may I ask for a review, please?

[marsskop]

Tests pass now:

$ go test                                                          
PASS
ok      software.sslmate.com/src/go-pkcs12      0.842s