Cherry pick new options methods from ngaut's fork

[kaedroho]

Part of #69

This pull request ports the following changes from @ngaut's fork:

• Add compression option https://github.com/ngaut/rust-rocksdb/pull/8
• Add max_bytes_for_level option https://github.com/ngaut/rust-rocksdb/pull/10
• Add disable_wal option https://github.com/ngaut/rust-rocksdb/pull/11
• Add report_bg_io and compression_per_level options https://github.com/ngaut/rust-rocksdb/pull/19
• Add set_bloom_filter and set_cache_index_and_filter_blocks to BlockBasedOptions https://github.com/ngaut/rust-rocksdb/pull/25
• Add set_level_zero_file_num_compaction_trigger option https://github.com/ngaut/rust-rocksdb/pull/30
• Add set_max_manifest_file_size option https://github.com/ngaut/rust-rocksdb/pull/33
• Add set_wal_recovery_mode option https://github.com/ngaut/rust-rocksdb/pull/34
  • follow up fix: https://github.com/ngaut/rust-rocksdb/pull/36
• Add enable_statistics and set_stats_dump_period_sec options https://github.com/ngaut/rust-rocksdb/pull/42
• Add set_num_levels option https://github.com/ngaut/rust-rocksdb/pull/45
• Add get_statistics option https://github.com/ngaut/rust-rocksdb/pull/49

The following changes will be dealt with separately

Add set_iterator_upper_bound to ReadOptions https://github.com/ngaut/rust-rocksdb/pull/31

This changes the ownership of ReadOptions which we would like to avoid if possible

Add set_compaction_filter option https://github.com/ngaut/rust-rocksdb/pull/43

This change is big and porting it would be difficult as it's based on lots of refactoring

[kaedroho]

Just added docstrings for most of the methods (including existing ones). These were copied from the RocksDB code itself (https://github.com/facebook/rocksdb/blob/869ae5d7868d7d48dd55b7be02a23581101ed69a/include/rocksdb/options.h).

They're quite inconsistent and have no examples, but it's much better than before!

[spacejam]

Looks great! Thanks for this! I'll remove the 1.2 failing test now, since that's pretty ancient by rust standards now.

[kaedroho]

Thanks for the quick review!