Add set_expires_in and set_issued_now methods

[bugdea1er]

Hi there!

I added set_expires_in and set_issued_now method for the jwt builder.

• set_issued_now sets a "iat" claim by calling system_clock::now() by itself;
• set_expires_in sets a "exp" claim by adding the given duration to the "iat" claim system_clock::now()

Common usage of the jwt builder is:

• create a builder
• set the "iat" claim at the "now" moment
• set the token expiration timeout

This should make it a little bit more convenient.

Update: builder now has the Clock template parameter just like the verifier

[bugdea1er]

Should I update examples or README with the new methods?