Fix missing binary encoding

Hello,

while trying to find a tool to export my contacts from my Baikal server, I stumbled upon an exception using the backup option of pyCardDAV:

###### Error Output
>Traceback (most recent call last):
>  File "/home/tobias/git/pycarddav/bin/pc_query", line 138, in <module>
>    query(conf)
>  File "/home/tobias/git/pycarddav/pycarddav/controllers/query.py", line 60, in query
>    action(my_dbtool, search_string, conf)
>  File "/home/tobias/git/pycarddav/pycarddav/controllers/query.py", line 79, in backup
>    vcf_file.write(vcard.vcf.encode('utf-8'))
>  File "/home/tobias/git/pycarddav/pycarddav/model.py", line 315, in vcf
>    collector.append(prop + types + ':' + line[0])
>UnicodeDecodeError: 'ascii' codec can't decode byte 0xff in position 0: ordinal not in range(128)

###### Steps to Reproduce
Backup a vCard that contains a binary Photo and its encoding is stated by an uppercase 'b', e.g.:
> PHOTO;TYPE=PNG;ENCODING=B:iVBOR[...]
>PHOTO;ENCODING=B;TYPE=JPEG;VALUE=BINARY:/9j/[...]

###### Explanation of the Problem
I think i could track it down to a rather trivial problem and I propose the changes in this request as a solution.

In *model.vcard_from_string()*, the photo value is being decoded to binary by *vobject.readOne()*. The following call to *vcard_from_vobject()* tries to encode it again.

I found that in *vcard_from_vobject()*, the *line.ENCODING_paramlist* call may return the encoding parameter as uppercase or lowercase 'b', depending on the entry in the given vCard. Therefore, the match for *[u'b']* is not sufficient to catch those with 'B'-parameter, which causes that the value is not being encoded. This leads to the exception, e.g., when being written to a file.

Is there a particular reason, why *vcard.serialize()* method of *vobject* isn't used directly?

###### Related

In my opinion, this also solves the problem of #39, in which case the changes made in 07589c762582a75a977dae3cae3ce51f5c8e5c51 are no longer necessary.

###### Minor Point

I also found that, when backing up / exporting multiple contacts at once, entries are being concatenated immediately, resulting in

>END:VCARDBEGIN:VCARD

lines in the output file. With this request, the vCards are separated by a newline character.

Kind regards
Tobias Röttger