Restore ability to build liteloader mods in 1.7.2.

Also allows the modder to use any version of liteloader he/she wants.
To use, add `setExt "liteloaderVersion", version` to the minecraft block of the buildscript.

Currently, the latest version of LiteLoader for 1.7.2 (1.7.2_05) doesn't have an mcp jar.  This will allow the modder to point to a different version of LiteLoader when that happens.  For example, to build a liteloader mod in 1.7.2, you would use this:
```
apply plugin: 'liteloader'
minecraft{
    version = "1.7.2"
    runDir = "eclipse"
    setExt "liteloaderVersion", "1.7.2_04"
}
```
Additionally, if an invalid version is given, it will spit out a list of accepted versions to choose from.