Don't override source if folder exists

* Bump SDK version to 6.1
* Minor improvements on logging
Updated to build curl 7.29.0
Add support to build with debug symbols
Update README to include dependency on Ruby 1.9

Closes #17
Update libcurl to version 7.30.0
Update libcurl to version 7.31.0
Compatibility changes for Xcode 5 Command Line Tools

Basically just renaming 'llvm-gcc-4.2' to 'gcc'
Update base SDK and accept arm64 as valid arch

Scripts still can't compile for arm64 but at least now they accept the
architecture as valid :\