Linux 64 bit: added x86_64 build (for Firefox 19)

Get architecture with uname -m and build libwebcl64.so
if x86_64 detected. Updated chrome.manifest accordingly.
Merge pull request #3 from Ozone77/linux64

Linux 64 bit: added x86_64 build support (including build for Firefox 19)
Updated for Firefox 20.
Linux32 build for 1.0.3.2013-04-03
Win32 build for 1.0.3.2013-04-03 (Firefox 20)