Searchable content has now better preprocessing. Migration included. Fixed #62

Preprocessing operations:
 - Strip HTML tags
 - Remove newlines, returns and tab characters
 - Trim double and trailing spaces
 - Convert to lower case
 - Remove URLs
 - Remove email addresses

Migration file 0010 includes forward and backward conversions of the data,
independant of the models' implementation in the loaded version of the code.
Merge remote-tracking branch 'origin/master'