Added a bunch of MBR type specific constructors

AAP, AST, DM, CLASSIC, MODERN
Added .toBuffer() to MBR.Partition()
Restructured MBR constructor and static methods

- Added MBR.parse( buffer )
- Added MBR.detectType( buffer )
- MBR ctor now basically just calls MBR.parse()
  after figuring out the type of record
Updated README: Added .toBuffer() to MBR methods
Updated test/: Removed check for .partitionCount

Deprecated.