Added Amazon AWS KMS encryption support

[mikljohansson]

Enables encryption and decryption using AWS Key Management Service.

AWS Key Management Service (KMS) is a managed service that makes it easy for you to create
and control the encryption keys used to encrypt your data, and uses Hardware Security Modules
(HSMs) to protect the security of your keys.

For example

echo -n secret | ./secretary encrypt --kms-key-id=12345678-1234-1234-1234-123456789012

Decryption with KMS can be done either locally using secretary decrypt if the current instance has access to the KMS key via an IAM role. Or in the case of containers by talking to the secretary daemon that itself executes on privileged instances that has the IAM role. Using the daemon avoids having to give all Mesos slaves access to all the KMS keys.

[codecov-io]

Current coverage is 54.54%

Merging #6 into master will decrease coverage by -0.18% as of 019bf74

@@            master      #6   diff @@
======================================
  Files            7       8     +1
  Stmts          402     451    +49
  Branches        62      69     +7
  Methods          0       0       
======================================
+ Hit            220     246    +26
- Partial         25      30     +5
- Missed         157     175    +18

Review entire Coverage Diff as of 019bf74

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[mikljohansson]

Also fixed problem where no more than 2 envelopes were processed in each string. regex.FindAllString(.., 2) had a limit of 2 enveloped, changed to -1 meaning unbounded number of envelopes can be extracted

[alexandernilsson]

LGTM