hash User API Keys.

[m-javani]

Hashing user API keys

This pull request is to store the hashed version of User API Keys.

Description

This pull request deals with issue #242 and applies the following changes:

• User API keys are hashed before being inserted into the database.
• UI no more shows the keys. Users still can delete the keys by key name

[loleg]

Test failing with error:

--- FAIL: TestUser (4.75s)
    user_test.go:64: 
        Unexpected non-nil/non-zero argument at index 1:
        	<*errors.errorString>: &errors.errorString{s:"authority missing"}
FAIL
FAIL	github.com/ProxeusApp/proxeus-core/storage/database

I am going to close this and re-open after doing the fix in another branch.