Tests for saving to PDF

Some simple tests for saving as PDF. 

For a given image mode, save to PDF and assert (a) the file exists and (b) is non-zero. Doesn't check the PDF contents but at least makes sure the code is exercised from beginning to end and that it produces something.

This has found two problems, ~~and the problematic tests have been commented out with FIXMEs so the build doesn't fail~~ which have been fixed.

~~1) Saving a P-mode image results in a TypeError:~~
```
  File "/home/travis/build/hugovk/Pillow/PIL/PdfImagePlugin.py", line 108, in _save
    colorspace = colorspace + b"> ]"
TypeError: Can't convert 'bytes' object to str implicitly
```
~~e.g. https://travis-ci.org/hugovk/Pillow/builds/24915249~~

~~2) Saving a 1-mode image triggers a DeprecationWarning, and any warnings cause the build to fail:~~
```
/home/travis/build/hugovk/Pillow/PIL/PdfImagePlugin.py:147:
DeprecationWarning: tostring() is deprecated. Please call tobytes() instead.
  data = im.tostring("raw", "1")
```
~~e.g. https://travis-ci.org/hugovk/Pillow/builds/24916085~~

(This PR branch inadvertently contains PR https://github.com/python-imaging/Pillow/pull/655, let me know if you'd like me to remove it from this PR.)