docs: Add architecture doc

Initial framework and design
Merge pull request #1 from unihorn/31

docs: Add architecture doc
feat: add build
feat: add test
Merge pull request #2 from unihorn/30

add build and test
feat: add Depot module
Merge pull request #3 from unihorn/30

feat: add Depot module
feat: Add pkix module

pkix module is responsible for certificate and key in PKIX.

For now, it could create a brand-new CA.
Merge pull request #4 from unihorn/30

feat: Add pkix module
refactor(depot): use tag instead of name

Compared to name, tag takes permission into account for each
data unit.

Add `Check` function to check validity of tag.
refactor(pkix): remove pem.Block from member vars

The object has no need to keep PEM-format representaion, because
this could be generated by itself.
feat(pkix/csr): Add certificate signing request
feat(pkix/cert): add creation of certificate for host
chore(depot): add comment for Tag
chore: add github.com/jstemmer/pkcs10
chore(pkix): add some comments
Merge pull request #6 from unihorn/31

feat(pkix): add certificate signing request and certificate for host
Merge pull request #5 from unihorn/30

refactor(depot): use tag instead of name
chore(pkix): add and rename some function
chore(depot): add List and GetFile function