Improved the INSTALL documentation.

Fixed typos, wording, grammar and the default prefix for manual
installation which should _never_ be /usr, as this is reserved for
package manager managed files only. Manual installations should always
go to a local (~/my-builds) directory or system-wide to /usr/local.
Fixed several typos and addressed some issues.

/etc/passwd should never be read directly, but instead getpwent() should
be used to get a list of users.
The GLX requirement for QtQuick2 can be fulfilled by using the upcoming
xorg-server-1.15.0, which is now also documented.
Corrected usage of 'getent passwd' and replaced it with 'getpwent()'.
Merge pull request #124 from eliasp/docs

Some updates/improvements/etc. for the documentation.