getLogicalLines parsing needs a rewrite

[da4089]

Quick Summary
The parser in vobject/base.py's getLogicalLines() is pretty messsy:

Context

• vObject version
• Python version
• Operating system and version

Description
The code is unclear and likely has poor performance:

• Should it really be split into QUOTED-PRINTABLE and not quoted branches?
• Can we avoid using StringIO buffers?
• In particular, the truncation of the trailing '=' for quoted printable content is ugly

A general re-think, rewrite, and clean up is needed for this function.