Fix some erors in Info.plist

This fixes a couple of issues, none too major:
- The MegaGlest executable is `megaglest` so that string was updated
- MG_VERSION_H_CONTENTS *includes* the file with the version in it (which I
  didn't realise when I changed it last time as I couldn't get things building
  still). This changes to using VER_* variables
- Update copyright to 2015 (I'm gambling on the project lasting another 2 weeks)
- Change CFBundleSignature to MGGL; a four letter contraction of MeGaGLest
Change Mac specific CMakeLists for CPack

This commit includes several fixes and some shuffling of entries to improve
readability. The major changes are Removing MG_VERSION_H_CONTENTS which
*includes* the file with the version in it in favour of VER_* variables and
removing the CPack include at the end of the file.

CPack has to be included after all variables are set and this file is sourced
before all the common options are configured.

One more fixme is added - this and other runtime issues will have to be dealt
with when i test the packaging, so far I've only got it to build.
Split up CPack generator configuration

I've taken the liberty of splitting up the CPACK_GENERATOR test so shared
configuration and win32 configuration are outside that test. The existing
grouping forced duplication of variables in the mac config file and required
the cpack generator to be unset, then set to deb, then set to nsis on win32.

These changes should help make things much more understandable.
Change coment on Apple test

This is just a niceity ; the comment no longer accurately captured what the
code was doing.
Delete any Megaglest DMG

Now I've fixed the dmg generation cleanup.sh is deleting a file that isn't
being created. Now we delete anything MegaGlest-*.dmg which should cover it.