Add c-ares and curl to fix #75
Merge pull request #80 from Metaswitch/iss75

[Reviewer: Andy] Add c-ares and curl to fix #75
Use rapidjson as a submodule
Merge pull request #81 from Metaswitch/rapidjson

[DON'T MERGE] [REVIEWER: AMC] Move to rapidjson submodule
Update cpp-common
Merge pull request #82 from Metaswitch/update_cppcommon

No reviewer: Update cpp-common
Change Metaswitch submodule refs to ClearwaterCore