Find-AVSignature Performance Improvements

Updated code to use [System.IO.FileStream] class with a buffer (64kb
default) to greatly increase performance, especially when handling large
files.

Updated $EndBytes validation logic to change it to a valid value rather
than throw an error.
Code cleanup based upon discussion comments

Added comment-based help parameter, added additional parameter
validation, piped ReadStream.seek command to out-null to prevent output
to stdout.
Merge pull request #3 from garignack/master

Merged Find-AVSignature performance improvements from garignack. Thanks, garignack!