bump to 0.52

bump to 0.52

This is primarily a feature release, but contains a fair few bug
fixes, new features:

- ability to read and write MS Windows ICO and CUR files

- you can now add file format plugins to support new file formats

- add POD coverage tests

- setcolors() and addcolors() now accept color names and so on instead
  of requiring Imager::Color objects.

- flood_fill() can now fill to a specified border color instead of
  just to the area the same color as the seed.

Bug fixes:

- bounding_box for the T1 driver wasn't converting UTF8 to ascii when
  calculating the advance width.

- bounding_box for the T1 driver wasn't including leading and trailing
  spaces in the bounding box as the other drivers did, it also produced
  strange results for empty strings or strings containing only spaces

- when reading CMYK jpeg images they were being transferred to the
  image object as is, producing a four channel image.  It only looked
  ok due to an old still unfixed Photoshop bug.  We now convert from
  the inverted CMYK that photoshop (and Corel for example) produce
  into RGB.

- reading a CYMK TIFF would result in a 4 channel image, reading any
  image with more than 4 channels (eg. RGB with 2 alpha channels)
  would result in an error.

- added /usr/local/include to the default include search path, since
  we were already searching /usr/local/lib for libraries.

And various minor fixes and documentation updates.