allow to duplicate Metadata

[bollenn]

allow to duplicate Metadata sections to remove error introduced in #71

*** Settings ***
Metadata    Version        2.0
Metadata    More Info      For more information about *Robot Framework* see http://robotframework.org
Metadata    Executed At    ${HOST}

would generate 2 errors like:

E: 11, 0: Setting 'Metadata' used multiple times (previously used line 10) (DuplicateSettingsInSuite)

Fixes #78

[bollenn]

@boakley

[bollenn]

@boakley @fperrin ready for review