Improved code for export

[axnow]

This fix resolves problem with exporting files that were located on GDrive, as they may not have url_download defined.
It also stores dms (json and content) so the export is full.
Exported JSons are now pretty-printed, which helps debugging.

[axnow]

Hi, back on track so about previous comments.

As for code to skip in case where no url_download is present: I have encountered following case (PDF on GDrive):

{'channels': [],
 'comments_count': 0,
 'created': 1440752344,
 'display_as_bot': False,
 'editable': False,
 'external_type': 'unknown',
 'filetype': 'pdf',
 'groups': ['XXXXXXXXX'],
 'id': 'XXXXXXXXX',
 'ims': [],
 'is_external': True,
 'is_public': False,
 'mimetype': 'application/pdf',
 'mode': 'external',
 'name': 'Getting Started on Slack.pdf',
 'permalink': 'https://XXXXX.slack.com/files/XXXXXX/XXXXXXXX/XXXXXXX.pdf',
 'pretty_type': 'PDF',
 'public_url_shared': False,
 'size': 904843,
 'timestamp': 1440752344,
 'title': 'Getting Started on Slack.pdf',
 'url': 'https://drive.google.com/file/XXXXXX?usp=drivesdk',
 'url_private': 'https://drive.google.com/file/d/XXXXX/view?usp=drivesdk',
 'user': 'XXXXXXX',
 'username': u''}

Aparrently any gdrive file will cause this kind of issue, so I decided to go for this check.

I have fixed (by ignoring ;-) ) the problem with no 'text' key. A code to handle different message types should be created (cooperation would be welcome).

I have changed location of default CSS file (now it's in main directory) and it is copied into html directory.