Fixed some smaller formatting issues.

NewSteamMethod is now always part of a function, so that the actual Method of
type SteamMethod is only created if it is acutally required for a request and
not upon package initialization.
Additionally I changed some mixed case formatting in json response structs.
Added new method for steamId creation.

Now a steamId can also be created if the vanity url is known. Adapted
ResolveVanityUrl to return an error if the vanityUrl is not used by any Steam
user instead of nil.
Added first two Dota related API calls.

GetMatchDetails and GetMatchHistory were added to the steamapi. Further API
calls will follow as soon as I find time for them.