Added nice OS X install script, fixed Linux install.sh, fixed make uninstall OS X, added nicer uninstall scripts (uninstall.sh) and updated compilation instructions

The OS X install script is a .command rather .sh as this enables the user to double click without having to run "sh ./install.sh" in Terminal.

Linux's install.sh was installing to the incorrect place and was not installing its dependencies. This was because the commit adding this script was pushed after I made those changes. This has now been fixed.

"make uninstall_osx" calls Linux's uninstall as the uninstall sequence for both OSes are identical. The reason its two separate command was simply to make everything the same and easier for people. It would be odd if _linux and _osx was appended to everything but the uninstall script. Because uninstall was renamed to uninstall_linux, uninstall_osx broke as it was still calling uninstall. This has now been fixed.

I've also added nicer uninstall scripts. Because the make system was removed from the packaged releases then there was no easy way to uninstall gmosh. This has been added now.

Finally, the READMEs have been updated to direct people wanting to compile gmosh to use "make install_linux" rather than the older "make install".

### Packaged releases

I've marked these as v0.92 but you can rename them to whatever version you want.

* OS X release:  
http://cl.ly/2w3U3k0c2S06/download/gmosh_osx_v0.92.tar.gz
* New Linux release (fixed install.sh, added uninstall.sh, readme update):  
http://cl.ly/1s0G1X1x2W3C/download/gmosh_linux_v0.92.tar.gz