update unimod.xml with newest version from unimod.org

The Unimod curators updated recently their database with mods for heavy methyl SILAC, including the new unimod # 1414 [1].  This pull request replaces our unimod.xml with the latest version from Unimod.org and fixes #987.

The changes are substantial. The last update dates back to 8 August 2010 [2]. Could we merge and then apply the 2012 changes from @timosachsenberg again?

[1] http://www.unimod.org/modifications_view.php?editid1=1414
[2] https://github.com/OpenMS/OpenMS/commits/develop/share/OpenMS/CHEMISTRY/unimod.xml