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.

port/port_android.h

@@ -78,6 +78,9 @@ class CondVar {
 // On ARM chipsets <V6, 0xffff0fa0 is the hard coded address of a 
 // memory barrier function provided by the kernel.
 typedef void (*LinuxKernelMemoryBarrierFunc)(void);
+// TODO(user): ATTRIBUTE_WEAK is undefined, so this fails to build on
+// non-ARMV6_OR_7. We may be able to replace it with __attribute__((weak)) for
+// older ARM builds, but x86 builds will require a different memory barrier.
 LinuxKernelMemoryBarrierFunc pLinuxKernelMemoryBarrier ATTRIBUTE_WEAK =
     (LinuxKernelMemoryBarrierFunc) 0xffff0fa0;
 #endif