Using smart pointers to manage ownership of block allocations

This is a first pass at a solution to bug #222, a merge request opened for initial discussion of a solution.

Unit tests crash due to a double delete. In the constructor for a block, one passes in a BlockContents object. The Block's data_ field is set to the underlying Slice's pointer to the data. When passing in a BlockContents object that is allocated on the stack, the unique_ptr deletes the previously allocated memory once it goes out of scope, leaving the Block with a dangling pointer. An example can be pulled from block_based_table_builder.cc in BlockBasedTableBuilder::InsertBlockInCache:

```c++
[...]
std::unique_ptr<char[]> ubuf(new char[size+1]);
ubuf[size] = type;

BlockContents results;
Slice sl(ubuf.get(), size);
results.data = sl;
results.cachable = true;
results.allocation = std::move(buf);
results.compression_type = type;

Block *block = new Block(results);
[...]
```

I'm not sure of the best way to proceed from here, but my gut tells me that sticking a std::unique_ptr in here will end up touching a lot more code than initially expected.
`heap_allocated` should not be part of the struct, else what's the point?