0.4.0

CHANGES:

• support EC (P-256, P-384, P-521) account keys (@reynir @hannesm)
  (reported in #24 by @dinosaure)
• allow key_type to be passed into the alpn_solver (@hannesm)
• add RFC 7520 test cases (@reynir @hannesm)
• remove astring dependency (@hannesm)
• bugfix: "orders" field in account is Uri.t option, not a list (@hannesm)
  (reported in #27 by @torinnd)