More functions

[yancouto]

Depends on #6

Added many new functions to luasteam. Mostly functions I'm using at MarvInc, so they relate to Workshop stuff. All of them have documentation.

Functions added:

• ISteamUGC
  • createItem
  • startItemUpdate
  • setItemContent
  • setItemDescription
  • setItemPreview
  • setItemTitle
  • submitItemUpdateResult
  • getNumSubscribedItems
  • getSubscribedItems
  • getItemState
  • getItemInstallInfo
  • getItemUpdateProgress
  • startPlaytimeTracking
  • stopPlaytimeTracking
  • stopPlaytimeTrackingForAllItems
• ISteamUser
  • getPlayerSteamLevel
  • getSteamID
• ISteamUtils
  • getAppID
• Extra (not originally on the SDK)
  • parseUint64

Other changes:

• uint64's (leaderboard handles, steam ids, etc.) can now be compared with equality and converted to and from strings.
• Improved uint64 documentation

[yancouto]

(Rebased on top of #6, is this better than rebasing on master?)

[rilifon]

Rebase it with master before merging