Code Review: Regex2

tfpt review “/shelveset:Regex2;REDMOND\tomat”

Implements regex pattern rewriter: some escape sequences that are valid
in Ruby are not valid in .NET regex and need to fixed in the pattern.

Rough implementation of Regexp::union, will finish it next checkin.

Fixes bugs:
[ ironruby-Bugs-20205 ] Regexp - unexpected escape sequence _
[ ironruby-Bugs-20275 ] raise vararg overloads not implemented

Fixes TCPServer to allow accept w/o previous select.

Tomas