Add TlsHelper (designed to be used with sks-keyservers.net)

We can't use the AndroidPinning library for this, because it requires the certificate to be signed using a system CA, sks-keyservers.net uses there own CA
Add TlsHelper (designed to be used with sks-keyservers.net)

We can't use the AndroidPinning library for this, because it requires the certificate to be signed using a system CA, sks-keyservers.net uses there own CA
Add hkps support for sks-keyservers.net
Few fixes on hkps

TODO: Fix add as well
Merge branch 'hkps-by-default' of github.com:mar-v-in/open-keychain into hkps-by-default
Rewrote hkp add to work with hkps as well.
Also removes HttpClient dependency (deprecated for android)
Use URLEncoder instead of Uri.encode
Update Keyservers to use hkps as needed
Fix sks-keyservers.net sub pool
Merge branch 'master' into hkps-by-default
Move sks-keyservers.net CA into assets folder
Merge pull request #690 from mar-v-in/hkps-by-default

HKPS by default