Added schema caching to Schema.cs
Added schema caching to Schema.cs

How it works:
Schema web connection is opened and headers read.
If cache file is not present or its creation time is older than "Last-Modified" of web schema Schema is pulled from api.steampowered.com, put into cache file and  cache file creation time is set equal to Schema Last-modified.
If cache file is present and is up-to-date the Schema is downloaded from the file.
Update SteamTrade/Schema.cs

Fixed indentation, changed cache file name.
Merge pull request #131 from lazy1/master

Schema caching was added to SteamTrade/Schema.cs