rename package `awsecret` to `aws` (minio#15)

This commit renames the `awsecret` package
to make the API more idiomatic.

Before, `awsecret` should only contain types
and functions related to the AWS SecretsManager.
Therefore, it contained the generic `KeyStore` type.

Now, the package `aws` groups all AWS services that
can be used as key store. So, there is a `aws.SecretsManager`
type that implements a key store using the AWS SecretsManager
as storage.

implement audit log tracing (minio#12)

This commit adds support for tracing
audit log events.

Therefore, this commit removes the global
HTTP write timeout. The problem with server-wide
timeouts is that as a HTTP handler you cannot
adjust/opt-out of the timeout. This makes it
impossible to implement long-running server tasks
like tracing logs.

Instead, this commit implements a custom timeout
handler - similar to https://golang.org/pkg/net/http#TimeoutHandler
The custom timeout handler and Go's TimeoutHandler
differ in implementation details. See documentation.

Now, each HTTP handler that requires a (write) timeout
must be wrapped by a timeout handler. The server-wide
write-timeout is explicitly set to 0.

Further, this commit implements a audit log tracing CLI
that prints a human-readable text output to a TTY and JSON
otherwise.

Add Dockerfile to containerize kes

adjust env. variables to reflect project name change

This commit changes the env. variable prefix
from `KEY_` to `KES_`.

rename `keys` to `kes`

restrict identity assignment on a per-policy basis

This commit allows policies to restrict the assignment
of identities. Now, a policy can specify that you can
only assign identities to specific policies but not to
any policies.

rename package import path

add AGPL license file and license header

add AGPL license file and license header