3.0 release

[ilanschnell]

In this PR we:

• remove Python 2.7 support
• .decode() returns iterator - equivalent to .iterdecode()
• .search() returns iterator - equivalent to .itersearch()
• remove util.rindex(), use .index(..., right=1) instead
• remove util.make_endian(), use bitarray(..., endian=...) instead
• Remove hackish support for bitarray() handling unpickling, see detailed explaination in #207. This will close #206.