Check if file is PDF by magic number. Closes #98

It only reads the first line of the file and compares it with a regex.

According to the [PDF reference][1] (page 92) the first line shall contain any of this strings:

```
%PDF−1.0 
%PDF−1.1 
%PDF−1.2 
%PDF−1.3 
%PDF−1.4 
%PDF−1.5 
%PDF−1.6
%PDF−1.7
...
```

[1]: http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/devnet/pdf/pdfs/pdf_reference_1-7.pdf

Related to #48, #50 (`.skip_ensure_pdfs` should be obsolete) and #98