I just ran the Homebrew script as described in the installation page. I then ran brew doctor in Terminal, and it returned several errors. I'm not sure how to fix those errors, please help.

brew doctor

Error: Some directories in /usr/local/share/man aren't writable.
This can happen if you "sudo make install" software that isn't managed
by Homebrew. If a brew tries to add locale information to one of these
directories, then the install will fail during the link step.
You should probably `chown` them:

    /usr/local/share/man/de
    /usr/local/share/man/de/man1
Error: You have Xcode 4.2, which is outdated.
Please install Xcode 4.3.
Error: Unbrewed dylibs were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.

Unexpected dylibs:
    /usr/local/lib/libcdt.5.dylib    /usr/local/lib/libcgraph.6.dylib    /usr/local/lib/libgraph.5.dylib    /usr/local/lib/libgvc.6.dylib    /usr/local/lib/libgvpr.2.dylib    /usr/local/lib/libpathplan.4.dylib    /usr/local/lib/libxdot.4.dylib
Error: Unbrewed .pc files were found in /usr/local/lib/pkgconfig.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.

Unexpected .pc files:
    /usr/local/lib/pkgconfig/libcdt.pc    /usr/local/lib/pkgconfig/libcgraph.pc    /usr/local/lib/pkgconfig/libgraph.pc    /usr/local/lib/pkgconfig/libgvc.pc    /usr/local/lib/pkgconfig/libgvpr.pc    /usr/local/lib/pkgconfig/libpathplan.pc    /usr/local/lib/pkgconfig/libxdot.pc
Error: /usr/bin occurs before /usr/local/bin
This means that system-provided programs will be used instead of those
provided by Homebrew. The following tools exist at both paths:

    2to3

Consider amending your PATH so that /usr/local/bin
is ahead of /usr/bin in your PATH.