add first draft of _Link class
Implement pure-perl Cursor and connections

Also:

* adds better diagnostics to BSON errors by providing a stack trace when
  an exception is thrown.

* throws errors when queries fail

* replace connection holder with a direct proxy of master's link

A number of tests have been marked TODO.  We will fix them once we
implement a proper cluster/server abstraction in accordance with the
cluster monitoring spec.
Remove unused C and XS files
support protocol pack/unpack back to 5.8, except on big-endian platforms

Big-endian platforms require Perl 5.10 for proper endianness modifiers
on pack.