Changes to `valid_size()`
* accept common page sizes, such as letter and a4.
* parse dimensions of format: AuxBv#, where A is width, u is units, B is height, v is units, # are options.
* accept units: in, cm, mm, pt
Changes to `convert()`:
* resize pages based on dpi calculations
* default resize images into page size (like default resize in imagemagick)
* implement exact resizing (ignore dpi; equiv to ! in imagemagick)
Created `get_ndpi()`:
* provides dpi for page resizing
* implement fill resizing (equiv to ^ in imagemagick)
Other changes:
* default dpi in global variable
CMYK TIFFs and JPEGs both work. CMYK JPEG2000 images have not been
tested. Adobe Photoshop and some other software generate inverted CMYK
JPEGs. The image is assumed to be inverted if the "Adobe" (APP14) tag is
present. Images can be forced inverted with `-C "CMYK;I"`, and forced
not inverted with `-C CMYK`.
Pillow 2.4.0 added support for JPEG2000 using OpenJPEG 2.0. Because
Pillow calls the format JPEG2000 instead of JP2, we need to rename it to
enable the optimized code path. Should still be backwards compatible.
