Removed shell_exec instances and retrive timestamp from zip files (build.prop)

I suggest to use the timestamp contained in the zip file (ro.build.date.utc), because this can cause troubles in some circumstance, for example:

If you upload a file by hand on the server, the creation date of the file will be newer than the compillation date of the ROM and CMUpdater will believe that is a new version because CMUpdater compare the date contained in the build.prop (ro.build.date.utc).

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Removed shell_exec instances for md5 checksums of zip files, now uses only php's md5_file function, beacause  on the majority of php installations, shell_exec is disabled by default for security reasons.