Various fixes + graceful failire if dependencies are not met

1. Fixing regression introduced in 84d964 -- if we add every suffix of the symbol, we should also delete every suffix when we delete the symbol.
2. Graceful failure if L9, FuzzyFinder, leveldb or clang are not installed
3. Removed clang from the repository (it was not picked up by the plugin, and, moreover, was an old version incompatible with the plugin :))

Fixes #11
Prefer definition and exact matches when looking up symbols

Fixes #10
Fixes #7