Improve emails upon import completion

I've added several cases to these emails so the message is better for the users:

- Single file [SUCCESS] 
subject: Your CartoDB table import just finished 
body: Your table *name* was successfully imported. 
call to action: View your table 
link: *table*  

- Single file [FAIL] 
subject: Your CartoDB table import just finished 
body: We couldn't import your table *name*. 
call to action: View your tables 
link: dashboard  

- Multiple 
subject: Your CartoDB tables import just finished 
body: Your tables import just finished. *imported_tables* out of *total_tables* were successfully imported. 
call to action: View your tables 
link: *first table*  

Before it was a generic "Table import finished" subject and "*imported_tables* out of *total_tables* were successfully imported." successful or not.

I need your :eyes:  here, @rafatower 
I don't quite understand the magic behind `after_filter`, but I'd write here:
```
  mail :to => user.email,
          :subject => get_subject
```
and below, make the method `get_subject` just return the local variable `subject`. But I might be wrong