Add limited folder rename support

Support a template string for renaming folders. This feature was requested
because some users have public and private folders with identical names.
VCardParser: IM and website: combine multiple values into one

Don't discard information if we can help it at all: collect all values found
for IM and website values and implode them into a single string.
VCardParser: use array accessor methods and NULL checks

The modern way of checking whether a parameter is set.
ZarafaAuthBackend: simplify to bare essence

Remove superfluous comments, log statements and other cruft, to strip the file
down to its essence.
Revert changes made for Sabre-VObject 3.0.0-alpha4

Sabre-VObject 3.0.0 supports arrays for passing multiple TYPE parameters,
whereas 3.0.0-alpha4 did not. Using the alpha4 method would now give a quoted
string, so use an array instead.
IM tags: encode and decode properly

Add lots of logic to encode and decode instant messaging tags properly. On
encoding from vCard to MAPI, add parts separated by semicolons. On decoding,
split the encoded string at the semicolons and check that we don't accidentally
introduce duplicates.
Website: decode combined string into multiple URL tags

Split the property string on semicolons and give each part its own URL tag.
VCardProducer: make extendedProperties a class member

Since we need this in virtually all methods, make it a class member, just like
we've done in VCardParser.
README.md: update for version 0.21
Update ChangeLog and config version to v0.21