Added bloom filter support.

In particular, we add a new FilterPolicy class.  An instance
of this class can be supplied in Options when opening a
database.  If supplied, the instance is used to generate
summaries of keys (e.g., a bloom filter) which are placed in
sstables.  These summaries are consulted by DB::Get() so we
can avoid reading sstable blocks that are guaranteed to not
contain the key we are looking for.

This change provides one implementation of FilterPolicy
based on bloom filters.

Other changes:
- Updated version number to 1.4.
- Some build tweaks.
- C binding for CompactRange.
- A few more benchmarks: deleteseq, deleterandom, readmissing, seekrandom.
- Minor .gitignore update.

db/table_cache.h

@@ -35,6 +35,15 @@ class TableCache {
                         uint64_t file_size,
                         Table** tableptr = NULL);
 
+  // If a seek to internal key "k" in specified file finds an entry,
+  // call (*handle_result)(arg, found_key, found_value).
+  Status Get(const ReadOptions& options,
+             uint64_t file_number,
+             uint64_t file_size,
+             const Slice& k,
+             void* arg,
+             void (*handle_result)(void*, const Slice&, const Slice&));
+
   // Evict any entry for the specified file number
   void Evict(uint64_t file_number);
 
@@ -43,6 +52,8 @@ class TableCache {
   const std::string dbname_;
   const Options* options_;
   Cache* cache_;
+
+  Status FindTable(uint64_t file_number, uint64_t file_size, Cache::Handle**);
 };
 
 }  // namespace leveldb