Allow launcher "Install folder" option to be relative

This fixes a problem encountered on Linux (but may effect other
platforms). Previously, giving a relative path to the "Install folder"
option on the Options tab of the launcher would produce a error in
Minecraft, as it would be passed a non-absolute path for its libraries.

Example:
Install folder...  set to  .feedthebeast
Press Launch

Produced shortly after launch:
java.lang.UnsatisfiedLinkError: Expecting an absolute path of the
library: .feedthebeast/MindCrack/minecraft/bin/natives/liblwjgl.so

Now Produces:
No error, apparently normal operation.